Q: What is the exact dose of Onsior for my cat?

The approved dose of robenacoxib for cats is 1.0 to 2.4 mg/kg orally once daily, or 2 mg/kg subcutaneously for injection. Your veterinarian will calculate the precise dose based on your cat's body weight and the condition being treated.

Q: Can I give my cat Onsior with food?

It is recommended to give Onsior on an empty stomach, either one hour before or two hours after a meal, because food significantly reduces its absorption. However, if your cat vomits on an empty stomach, you can give it with a small amount of food.

Q: How long does it take for Onsior to work in cats?

Robenacoxib reaches peak blood concentrations within 1 hour of oral administration. However, its full analgesic effect may take a few hours to become apparent.

Q: Is Onsior safe for cats with kidney disease?

Studies have shown that robenacoxib is well tolerated in cats with stable chronic kidney disease, with no significant increase in adverse events compared to placebo. However, it should be used with caution and under close veterinary supervision.

Q: Can I give Onsior with other pain medications?

No. Onsior should never be combined with other NSAIDs or corticosteroids, as this increases the risk of severe side effects. If additional pain relief is needed, your veterinarian may prescribe an opioid or other non-NSAID analgesic.

Q: What happens if I miss a dose of Onsior?

If you miss a dose, give it as soon as you remember, but skip it if it is almost time for the next dose. Never give a double dose to make up for a missed one.

Q: How long can a cat stay on Onsior?

For chronic conditions like osteoarthritis, robenacoxib has been studied for up to 12 weeks and has been shown to be safe. For acute conditions, it is typically used for a few days.

Q: What are the most common side effects of Onsior in cats?

The most common side effects reported are gastrointestinal, such as vomiting, diarrhea, and reduced appetite. In clinical trials, the incidence of these events was similar to that seen with placebo.

# Onsior Robenacoxib for Cats: Feline NSAID Safe Dosing Protocol ## Key Takeaways - Robenacoxib (Onsior) is a COX-2 selective NSAID for cats, with an oral dosage range of 1.0-2.4 mg/kg once daily and a subcutaneous perioperative dose of 2 mg/kg. Its high COX-2 selectivity (32:1 COX-1:COX-2 ratio) minimizes gastrointestinal and platelet adverse effects compared to non-selective NSAIDs. - Oral administration requires careful timing relative to meals; administer 1 hour before or 2 hours after feeding to optimize bioavailability (49% fasted, 10% fed), and the tablet should not be split or crushed. Injectable bioavailability is higher (~69-78%), and formulations are interchangeable without a washout period. - Robenacoxib is indicated for perioperative pain management (e.g., ovariohysterectomy, orthopedic surgery) and chronic osteoarthritis, demonstrating efficacy in reducing pain scores and improving mobility. Pre-emptive administration at anesthesia induction may preserve renal function better than administration at hospital admission. - While generally well-tolerated, caution is advised in cats with pre-existing renal disease; monitoring renal parameters (creatinine, urine specific gravity) is recommended, especially in geriatric patients. Robenacoxib is contraindicated in dehydrated, hypotensive, or hypovolemic cats due to increased risk of acute kidney injury. - Concurrent use with other NSAIDs or corticosteroids is strictly contraindicated due to additive risks of gastrointestinal ulceration and renal damage. Interactions with diuretics and ACE inhibitors require careful monitoring of renal function. - Owners must never administer human NSAIDs or acetaminophen to cats due to high toxicity. Immediate veterinary contact is required for signs of vomiting, diarrhea, anorexia, lethargy, increased thirst/urination, or jaundice. --- Robenacoxib, sold under the brand name Onsior, is a prescription non-steroidal anti-inflammatory drug (NSAID) approved for use in cats. It is a member of the coxib class, designed to provide pain relief and reduce inflammation with a high selectivity for the cyclooxygenase-2 (COX-2) enzyme while largely sparing the COX-1 enzyme, which is important for normal gastrointestinal and platelet function [1][2]. The registered dosage for cats is 1 to 2.4 mg/kg orally once daily, or 2 mg/kg subcutaneously for perioperative use [2][3]. This article provides a definitive, source-grounded protocol for the safe use of robenacoxib in cats, covering dosing, safety margins, renal considerations, and clinical applications. **Owner Triage Summary:** If your [cat](/knowledge/veterinary-medicine/clinical-methods/cat) has been prescribed Onsior, give the medication exactly as directed by your veterinarian. The typical dose is one tablet per day, and the tablet should not be split. Do not give this medication to a cat that is dehydrated, in shock, or has known kidney disease unless explicitly instructed by your vet. If your cat is undergoing surgery, the injection is often given at induction or before the procedure. Never combine Onsior with other NSAIDs or corticosteroids. If you miss a dose, skip it and resume the next day; do not double up. Contact your veterinarian immediately if your cat develops vomiting, diarrhea, loss of appetite, or unusual lethargy. ## At a Glance: Robenacoxib Dosing and Key Parameters | Parameter | Feline Value | Source Notes | | :--- | :--- | :--- | | **Oral Dosage (Tablets)** | 1.0 to 2.4 mg/kg, once daily | Approved range for pain and inflammation [2] | | **Injectable Dosage (SC)** | 2.0 mg/kg, once daily | For perioperative use, often given pre-surgery [3] | | **Duration (Osteoarthritis)** | Up to 12 weeks studied | Clinical trials show safety for chronic use [4] | | **COX-2 Selectivity (in vitro)** | IC50 ratio 32:1 (COX-1:COX-2) | High selectivity for the inflammatory enzyme [2] | | **Blood Half-Life** | < 2 hours (approx. 1.1-1.5 h) | Short in blood, but concentrates in inflamed tissue [5][2] | | **Bioavailability (Oral, fasted)** | ~49% | Increases to ~69% when given subcutaneously [6] | | **Excretion** | Mainly fecal (72-78%) | Mixed excretion via bile and urine [7][2] | | **Renal Safety** | Minimal effect on GFR in healthy cats | Caution advised in cats with CKD; monitor closely [8][9] | | **Drug Interactions** | Avoid other NSAIDs, corticosteroids, diuretics | Risk of acute kidney injury increases with combination [9] | ## Pharmacology and Mechanism of Action Robenacoxib is a highly selective COX-2 inhibitor. In cats, the in vitro IC50 ratio for COX-1:COX-2 is 32:1, meaning it is 32 times more selective for COX-2 than COX-1 [2]. This selectivity is the cornerstone of its safety profile. COX-1 is a constitutive enzyme responsible for maintaining the gastric mucosal barrier, regulating renal blood flow, and promoting normal platelet aggregation. COX-2 is induced by inflammation and produces prostaglandins that mediate pain, swelling, and fever. By selectively inhibiting COX-2, robenacoxib can effectively manage pain and inflammation with a lower risk of gastrointestinal ulceration and bleeding complications compared to non-selective NSAIDs [1][5]. The pharmacokinetics of robenacoxib in cats are unique. The terminal half-life in blood is short, often less than 2 hours [5][2]. However, the drug is highly lipophilic and ion-trapped in acidic inflamed tissues. This means that while blood concentrations drop quickly, concentrations in inflammatory exudate remain high for over 24 hours, allowing for once-daily dosing [5]. A population pharmacokinetic model confirmed that no dose adjustment is needed based on age, gender, or bodyweight, although the study noted a high bioavailability of 78% for the subcutaneous route [10]. ## Safe Dosing Protocol: Oral and Injectable ### Tablet Administration Onsior tablets for cats are available in a 6 mg size. The approved oral dosage is 1.0 to 2.4 mg/kg once daily [2]. This range allows a veterinarian to select a dose based on the severity of the condition and the individual cat's size. For example, a 4 kg cat would receive a dose between 4 mg and 9.6 mg, which is typically achieved with one 6 mg tablet. The tablets are flavored and designed to be given once daily. **Feeding and Administration:** The absorption of robenacoxib is affected by food. When administered to cats that had been fed their entire daily ration, the bioavailability dropped to 10%, compared to 49% in fasted cats [6]. To ensure adequate absorption, it is recommended to administer the tablet either one hour before or two hours after a meal. However, if a cat vomits when given medication on an empty stomach, it may be given with a small amount of food, understanding that efficacy may be slightly reduced. The tablet should not be crushed or split, as this may affect the stability of the drug and the accuracy of the dose. ### Injectable Administration The injectable formulation of robenacoxib is administered subcutaneously at a dose of 2 mg/kg [3]. This route is primarily used in a clinical setting for perioperative pain management. The injection has a high bioavailability of 69 to 78% and reaches peak concentrations faster than the oral formulation [10][6]. It is often given at the time of premedication or at induction of anesthesia to provide pre-emptive analgesia [11][12]. ### Interchangeability of Formulations A safety study evaluated the interchangeable use of the 6 mg tablets and the 20 mg/mL injectable solution in healthy cats. The study involved cycles of seven days of oral administration followed by three days of subcutaneous administration. The results showed that this interchangeable use was well tolerated, even at doses up to three times the maximum recommended rate [13]. This provides clinical flexibility, allowing a veterinarian to start therapy with an injection and continue with oral tablets without a washout period, provided the total daily dose does not exceed 2.4 mg/kg. ## Evidence-Based Clinical Applications ### Perioperative Analgesia Robenacoxib is widely used for the control of perioperative pain in cats undergoing soft tissue and orthopedic surgery. - **Ovariohysterectomy (Spay):** A large placebo-controlled trial involving 349 cats undergoing onychectomy and ovariohysterectomy or castration demonstrated that cats receiving injectable robenacoxib (2 mg/kg SC) required significantly fewer rescue analgesics (19.7%) compared to those receiving placebo (41.7%) [3]. The drug was effective in controlling pain associated with soft tissue and bone surgery. - **Orthopedic Surgery:** In a comparative trial against meloxicam, robenacoxib administered at 2 mg/kg SC before surgery, followed by oral tablets (1-2.4 mg/kg) for approximately 9 days, was statistically non-inferior to meloxicam for controlling pain and inflammation associated with orthopedic surgery [12]. - **Timing of Administration:** A study on the timing of robenacoxib administration found that giving the drug at the time of anesthesia induction was associated with better preservation of renal function (as measured by plasma renin activity) compared to giving it at admission to the hospital [11]. This suggests that administering robenacoxib closer to the surgical event may be more physiologically appropriate. - **Gastrointestinal Surgery:** A retrospective study evaluated the use of robenacoxib immediately after gastrointestinal surgery. The study found that cats receiving robenacoxib did not have an increase in major postoperative complications (2.3%) compared to cats that did not receive it (12.6%) [14]. This provides evidence that robenacoxib can be used safely in this context, although it should be used with caution and under close veterinary supervision. ### Chronic Osteoarthritis (Degenerative Joint Disease) Osteoarthritis is a highly prevalent condition in cats, and robenacoxib is an effective option for managing the associated pain. - **Efficacy:** A pilot clinical trial in 109 cats with degenerative joint disease-associated pain showed that robenacoxib treatment significantly decreased owner-assessed disability (49% reduction) and improved temperament and happiness after 6 weeks of treatment [15]. While objective activity monitoring showed high variability, more cats treated with robenacoxib showed a greater than 10% increase in activity compared to placebo. - **Long-Term Safety:** A pooled analysis of four clinical trials involving 449 cats with chronic musculoskeletal disease evaluated the safety of robenacoxib administered for 4 to 12 weeks. The study found no significant difference in the number of cats with at least one adverse event between the robenacoxib (47.8%) and placebo (41.0%) groups [4]. This supports the safety of robenacoxib for longer-term use in a clinical population. ## Safety Profile and Renal Considerations ### Gastrointestinal and Platelet Safety The high COX-2 selectivity of robenacoxib translates into a favorable safety profile. In a study comparing robenacoxib to ketoprofen, ketoprofen significantly suppressed serum thromboxane (TxB2), a marker of COX-1 activity and platelet function, while robenacoxib did not [5]. This indicates a lower risk of bleeding complications with robenacoxib. Clinical trials have not shown a significant increase in gastrointestinal adverse events compared to placebo [8][4]. ### Renal Function and Chronic Kidney Disease The kidney relies on prostaglandins to maintain blood flow, particularly in states of hypotension or dehydration. NSAIDs can therefore affect renal function. - **Healthy Cats:** Studies in healthy cats have shown that robenacoxib has minimal effects on glomerular filtration rate (GFR). A study combining robenacoxib with benazepril (an ACE inhibitor) and furosemide (a diuretic) found that the combination was well tolerated with no negative effects on clinical chemistry or urinalysis variables [9]. - **Cats with CKD:** A specific safety study evaluated robenacoxib in a subgroup of 40 cats with concurrent chronic kidney disease (defined as serum creatinine ≥1.6 mg/dl and urine specific gravity <1.030). The study found no differences between the robenacoxib and placebo groups in terms of adverse events, body weight change, or serum chemistry variables over a 28-day period [8]. Another pooled analysis of cats with CKD found the relative risk of an adverse event with robenacoxib compared to placebo was 1.09, which was not statistically significant [4]. - **Clinical Caution:** Despite these reassuring findings, a slight increase in serum creatinine (by approximately 4.36 μmol/L) was observed in robenacoxib-treated cats compared to placebo in one large study [4]. While no adverse clinical effects were detected, this highlights the need for caution. Robenacoxib should be used with care in cats with pre-existing kidney disease, and it is contraindicated in dehydrated or hypotensive cats. Monitoring kidney function before and during treatment is recommended, especially in geriatric patients. ## Contraindications and Drug Interactions Robenacoxib should not be used in cats with: - Known hypersensitivity to the drug. - Pre-existing gastrointestinal ulceration or bleeding. - Significant renal or hepatic impairment, unless the potential benefits outweigh the risks and monitoring is in place. - Dehydration, hypovolemia, or hypotension, as this increases the risk of acute kidney injury. **Drug Interactions:** The concurrent use of robenacoxib with other NSAIDs or corticosteroids is strictly contraindicated due to the additive risk of gastrointestinal ulceration and renal damage. Caution is also advised when using robenacoxib concurrently with diuretics (like furosemide) or ACE inhibitors (like benazepril), as the combination can affect renal function, although one study showed it was tolerated in healthy cats [9]. Always provide a complete medication history to your veterinarian. ## Managing Acute Pain: A Comparative View While robenacoxib is a mainstay for pain control, other options exist. A study comparing robenacoxib to grapiprant (another NSAID) found both to be effective for controlling pain after ovariohysterectomy, with grapiprant showing non-inferiority to robenacoxib [16]. Another study comparing robenacoxib to tolfenamic acid found that robenacoxib provided superior pain control at 30 and 48 hours post-operatively [17]. These comparisons help veterinarians choose the best NSAID for each patient. ## Unsafe Home Remedies and Owner Responsibilities It is critical that owners never administer human NSAIDs (such as ibuprofen, naproxen, or aspirin) to cats. These drugs are extremely toxic to felines and can cause severe gastrointestinal ulcers, kidney failure, and even death. Acetaminophen (paracetamol) is also highly toxic to cats. Always use only veterinary-prescribed medications. When giving Onsior at home, follow these guidelines: - Administer the tablet directly into the mouth or hidden in a small treat, as directed by your vet. - Ensure your cat has access to fresh water at all times. - Monitor your cat for any changes in appetite, thirst, urination, or behavior. - Do not use Onsior for longer than prescribed without consulting your veterinarian. ## Prognosis and Prevention The prognosis for cats with acute pain or chronic osteoarthritis is generally good with appropriate NSAID therapy. Robenacoxib can significantly improve quality of life by reducing pain and improving mobility [15]. For chronic conditions, management often involves a multimodal approach, including weight management, environmental enrichment (e.g., providing easy access to food and litter boxes), and joint supplements, in addition to NSAID therapy. Prevention of conditions requiring NSAID use involves maintaining a healthy body weight, providing regular, low-impact exercise, and minimizing the risk of trauma. For surgical cases, the use of pre-emptive analgesia is a standard of care to prevent central sensitization and reduce postoperative pain. ## Limitations and When to Contact a Veterinarian This article provides a general overview of robenacoxib dosing and safety. It cannot predict how an individual cat will respond to therapy. Breed-specific sensitivities to NSAIDs are not fully characterized for robenacoxib in cats, and the available data does not support specific breed-based dose adjustments [10]. The information here is educational and is not a substitute for veterinary diagnosis or treatment. **Emergency Red Flags:** Contact your veterinarian immediately if your cat exhibits any of the following signs while on robenacoxib: - Vomiting (especially if repeated or contains blood). - Diarrhea, particularly if it is dark or tarry. - Loss of appetite lasting more than 24 hours. - Lethargy or weakness. - Increased thirst or urination, or a decrease in urine output. - Yellowing of the gums or skin (jaundice). ## The Clinical Rationale Behind COX-2 Selectivity in Feline Patients Understanding why robenacoxib is considered a safer NSAID option for cats begins with appreciating the distinct roles of the cyclooxygenase enzymes in feline physiology. The COX-1 enzyme is constitutively expressed in most tissues and is responsible for producing prostaglandins that protect the gastric mucosa, support normal platelet aggregation, and help maintain renal perfusion. COX-2, by contrast, is largely induced at sites of inflammation, where it drives the production of prostaglandins that sensitize pain receptors and promote vasodilation and edema. The in vitro IC50 ratio of 32:1 for COX-1 to COX-2 means robenacoxib is 32 times more selective for the inflammatory enzyme [2]. This selectivity is not merely a laboratory curiosity; it has direct clinical consequences. In a comparative study against ketoprofen, a non-selective NSAID, robenacoxib did not suppress serum thromboxane (TxB2), a marker of COX-1 activity and platelet function, whereas ketoprofen significantly did [5]. This finding is clinically relevant for surgical patients. Platelet function is essential for normal hemostasis, and a drug that preserves platelet aggregation reduces the risk of intraoperative and postoperative bleeding. For a cat undergoing an ovariohysterectomy or an orthopedic procedure, this translates into a more predictable surgical field and a lower likelihood of complications related to hemorrhage. The same study demonstrated that robenacoxib maintained its anti-inflammatory effect in inflammatory exudate for over 24 hours despite a short blood half-life, confirming that the drug's tissue distribution, not its blood concentration, determines its duration of action [5]. The clinical implication of this pharmacokinetic profile is that once-daily dosing is sufficient. Owners often worry that a drug with a short blood half-life might not provide round-the-clock pain relief. The ion-trapping mechanism, where the weakly acidic robenacoxib molecule accumulates in the slightly acidic environment of inflamed tissue, ensures that the drug remains at the site of action long after it has been cleared from the bloodstream. This is a key point to communicate to owners who may be tempted to give a second dose if they perceive their cat is still uncomfortable. Giving more than the prescribed dose does not improve efficacy; it only increases the risk of adverse effects. ## Diagnostic Workup Before Initiating Robenacoxib Therapy Before prescribing robenacoxib, a veterinarian should perform a thorough diagnostic evaluation to identify any underlying conditions that might increase the risk of NSAID-related complications. This is particularly important in geriatric cats, where chronic kidney disease (CKD), hepatic insufficiency, and hypertension are common comorbidities. The diagnostic workup should include a complete blood count, serum biochemistry profile, and urinalysis. The biochemistry profile should specifically evaluate blood urea nitrogen, serum creatinine, and liver enzymes such as alanine aminotransferase and alkaline phosphatase. A urinalysis is essential to assess urine specific gravity, which helps differentiate prerenal azotemia from intrinsic renal disease. A cat with a urine specific gravity below 1.030 in the presence of elevated creatinine may have underlying CKD, and the decision to use robenacoxib should be made with caution and with a plan for monitoring [8]. Blood pressure measurement is another critical component of the pre-treatment evaluation. Hypertension is common in older cats and can be both a cause and a consequence of renal disease. NSAIDs can further compromise renal blood flow in a hypertensive or hypotensive patient. If a cat is diagnosed with hypertension, it should be managed with antihypertensive therapy before or concurrently with NSAID treatment, and the combination should be monitored closely. The study combining robenacoxib with benazepril and furosemide in healthy cats found no negative effects on clinical chemistry or urinalysis variables, but this was in healthy animals [9]. In a patient with pre-existing renal compromise, the same combination could have different outcomes, and the risk-benefit ratio must be assessed individually. For cats presenting with lameness or suspected osteoarthritis, diagnostic imaging is recommended to confirm the diagnosis and rule out other causes of pain, such as fractures, neoplasia, or infectious arthritis. Radiography can reveal joint effusion, osteophyte formation, and subchondral bone sclerosis characteristic of degenerative joint disease. While radiographs are useful, they do not always correlate with the severity of clinical signs. Some cats with significant radiographic changes show minimal lameness, while others with mild radiographic changes are severely affected. This discrepancy underscores the importance of a thorough orthopedic and neurologic examination in addition to imaging. The diagnosis of osteoarthritis-associated pain in cats is often based on a combination of physical examination findings, owner-reported behavioral changes, and imaging results [15]. ## Owner Observation and Preparation for a Veterinary Visit Owners play a pivotal role in the safe use of robenacoxib. Their observations before, during, and after treatment can help the veterinarian make informed decisions about dosing, duration, and the need for additional diagnostics. Before the veterinary visit, owners should be prepared to provide a detailed history, including the onset and duration of clinical signs, any changes in appetite, thirst, urination, or behavior, and a complete list of all medications, supplements, and topical products the cat has received. This includes flea and tick preventives, which are often overlooked but can contain ingredients that interact with other drugs. Owners should also be encouraged to video-record their cat's gait at home. Cats often move more freely in their familiar environment than in the veterinary clinic, where they may be stressed and reluctant to move. A video of the cat jumping onto a sofa, climbing stairs, or playing can provide valuable information about the severity of lameness or stiffness. The owner-assessed disability questionnaire used in clinical trials of robenacoxib for degenerative joint disease includes questions about the cat's ability to jump, groom, and interact with family members [15]. Owners can be asked to rate these activities on a scale, which provides a semi-quantitative measure of treatment response. During treatment, owners should maintain a daily log of their cat's behavior, appetite, and any adverse events. This log should be shared with the veterinarian at follow-up appointments. Specific items to record include the time of medication administration, the cat's willingness to eat, the frequency and character of vomiting or diarrhea, and any changes in water consumption or urine output. A cat that suddenly stops eating or becomes lethargic may be experiencing an adverse drug reaction, and the medication should be discontinued and the veterinarian contacted immediately. The emergency red flags listed in the main article, including vomiting, diarrhea, loss of appetite, lethargy, increased thirst or urination, and jaundice, should be prominently displayed in the owner's mind. ## Evidence Limitations and Gaps in Current Knowledge While the existing clinical trials provide a solid foundation for the use of robenacoxib in cats, it is important to acknowledge the limitations of the evidence. Many of the pivotal studies were industry-sponsored and had relatively short durations, typically 4 to 12 weeks for chronic conditions [4]. The long-term safety of robenacoxib beyond 12 weeks is not well characterized. Osteoarthritis is a progressive, lifelong condition, and many cats will require pain management for years. Extrapolating the safety data from a 12-week trial to years of continuous use requires clinical judgment and regular monitoring. The study population in clinical trials is also not fully representative of the general feline population. Most trials enrolled adult cats with stable health, excluding those with significant comorbidities such as advanced CKD, hepatic disease, or cardiac disease. The safety study that included 40 cats with CKD is an exception, but even this study had a relatively short duration of 28 days [8]. The effect of robenacoxib on cats with International Renal Interest Society (IRIS) stage 3 or 4 CKD, or on cats with concurrent conditions such as diabetes mellitus or hyperthyroidism, remains largely unknown. These cats are often the ones that present with chronic pain, and the veterinarian must weigh the potential benefits of pain relief against the uncertain risks of NSAID therapy. Another limitation is the lack of breed-specific data. While a population pharmacokinetic model found no need for dose adjustment based on age, gender, or bodyweight, the study did not specifically evaluate breed differences [10]. Certain breeds, such as Siamese and Oriental Shorthairs, are known to have idiosyncratic reactions to some drugs, but whether this applies to robenacoxib is unknown. Until more data are available, the standard dose of 1 to 2.4 mg/kg once daily should be used for all breeds, with close observation for adverse effects. The evidence for the use of robenacoxib in cats with gastrointestinal disease is also limited. The retrospective study on gastrointestinal surgery found no increase in major postoperative complications in cats receiving robenacoxib, but this was a retrospective analysis with inherent limitations, including selection bias and incomplete medical records [14]. Prospective, randomized controlled trials are needed to confirm these findings. In the meantime, robenacoxib should be used with caution in cats with a history of gastrointestinal ulceration or inflammatory bowel disease. ## Perioperative Analgesia: A Deeper Dive into Clinical Decision-Making The perioperative period is a critical time for pain management, and robenacoxib has a well-established role in this setting. The decision of when to administer the drug is not trivial. The study comparing administration at the time of anesthesia induction versus at the time of admission to the hospital found that induction-time administration was associated with better preservation of renal function, as measured by plasma renin activity [11]. This finding has a physiological basis. Administering the NSAID closer to the surgical event ensures that the drug is present at the site of tissue trauma when the inflammatory cascade is initiated, providing pre-emptive analgesia. It also minimizes the duration of NSAID exposure before surgery, reducing the window for potential renal or gastrointestinal effects. For a cat undergoing an ovariohysterectomy, the standard protocol might involve administering robenacoxib at 2 mg/kg subcutaneously at the time of premedication, followed by oral tablets for 3 to 5 days postoperatively. The large placebo-controlled trial in 349 cats undergoing onychectomy and ovariohysterectomy or castration demonstrated that cats receiving robenacoxib required significantly fewer rescue analgesics (19.7%) compared to placebo (41.7%) [3]. This is a clinically meaningful difference. Rescue analgesia is not only uncomfortable for the cat but also prolongs recovery and may require additional drugs that have their own side effects. For orthopedic surgery, the comparative trial against meloxicam showed that robenacoxib was statistically non-inferior [12]. The protocol in this study involved a subcutaneous injection before surgery followed by oral tablets for approximately 9 days. This duration is longer than what is typically used for soft tissue surgery, reflecting the greater tissue trauma and longer recovery period associated with bone surgery. The veterinarian should tailor the duration of postoperative robenacoxib to the type of surgery and the cat's individual pain score. A cat that is painful on day 3 after an orthopedic procedure may require continued NSAID therapy, while a cat that is comfortable may be weaned off earlier. The use of robenacoxib in gastrointestinal surgery deserves special mention. Historically, NSAIDs were avoided in patients undergoing gastrointestinal surgery due to concerns about anastomotic leakage and delayed healing. The retrospective study that evaluated robenacoxib use immediately after gastrointestinal surgery found no increase in major postoperative complications (2.3%) compared to cats that did not receive it (12.6%) [14]. While these results are encouraging, they should be interpreted with caution. The study was retrospective, and the decision to administer robenacoxib was made by the attending clinician, which introduces potential bias. Cats that were perceived to be at higher risk may have been less likely to receive the drug. Nevertheless, the data suggest that robenacoxib can be used safely in selected cases of gastrointestinal surgery, provided the cat is hemodynamically stable and well hydrated. ## Chronic Osteoarthritis: Beyond the Clinical Trial Data Osteoarthritis is estimated to affect a significant proportion of the feline population, particularly older cats. The clinical signs are often subtle and may be attributed to normal aging by owners. Common signs include reluctance to jump, decreased activity, stiffness after rest, and changes in grooming habits. The pilot clinical trial in 109 cats with degenerative joint disease-associated pain showed that robenacoxib treatment significantly decreased owner-assessed disability (49% reduction) and improved temperament and happiness after 6 weeks of treatment [15]. This is a substantial improvement and highlights the impact that effective pain management can have on a cat's quality of life. However, managing chronic osteoarthritis requires more than just prescribing an NSAID. A multimodal approach is essential for optimal outcomes. Weight management is arguably the most important intervention. Obesity places additional stress on already compromised joints, and weight loss has been shown to improve mobility in arthritic cats. A gradual weight loss program, with a target of 0.5% to 1% body weight loss per week, is recommended. This should be combined with a measured feeding plan and regular weigh-ins. Environmental modification is another key component. Providing easy access to food, water, and litter boxes is essential for an arthritic cat. This may involve placing food and water bowls on the ground floor, using a litter box with a low entry, and providing ramps or steps to allow the cat to reach its favorite resting spots. Soft, padded bedding can also help reduce pressure on painful joints. These modifications are often simple and inexpensive but can make a significant difference in the cat's daily comfort. Physical therapy and controlled exercise are also beneficial. Gentle, passive range-of-motion exercises can help maintain joint flexibility. Encouraging short, frequent periods of activity, such as interactive play with a wand toy, can help maintain muscle mass without overexerting the cat. The use of joint supplements, such as glucosamine and chondroitin sulfate, is common, although the evidence for their efficacy in cats is limited. Omega-3 fatty acids, which have anti-inflammatory properties, may also be beneficial. These supplements should be used as adjuncts to, not replacements for, NSAID therapy. The decision to use robenacoxib long-term for osteoarthritis should be made in consultation with the veterinarian. The pooled analysis of four clinical trials involving 449 cats with chronic musculoskeletal disease found no significant difference in the number of cats with at least one adverse event between the robenacoxib (47.8%) and placebo (41.0%) groups [4]. This is reassuring, but it does not mean that adverse events do not occur. Regular monitoring, including physical examinations, serum biochemistry profiles, and owner-reported assessments, is recommended every 3 to 6 months for cats on chronic NSAID therapy. The dose should be reassessed at each visit, and the lowest effective dose should be used. ## Renal Considerations in Special Populations The kidney is a primary concern when using NSAIDs in cats, and this concern is amplified in certain populations. Geriatric cats are at increased risk of CKD, and the prevalence of CKD increases with age. The safety study that evaluated robenacoxib in 40 cats with concurrent CKD (defined as serum creatinine ≥1.6 mg/dl and urine specific gravity <1.030) found no differences between the robenacoxib and placebo groups in terms of adverse events, body weight change, or serum chemistry variables over a 28-day period [8]. This is encouraging, but it is important to note that the cats in this study had stable, compensated CKD. Cats with more advanced disease, or cats that are dehydrated or hypotensive, are at much higher risk. The mechanism by which NSAIDs affect the kidney is well understood. Prostaglandins, particularly PGE2 and PGI2, help maintain renal blood flow by causing vasodilation of the afferent arteriole. In a healthy, well-hydrated cat, this prostaglandin-dependent vasodilation is not critical. However, in a cat that is dehydrated, hypotensive, or has pre-existing renal disease, the kidney becomes dependent on prostaglandins to maintain adequate perfusion. When an NSAID inhibits COX enzymes, prostaglandin production decreases, leading to afferent arteriolar constriction, reduced renal blood flow, and potentially acute kidney injury. This is why it is absolutely contraindicated to administer robenacoxib to a cat that is dehydrated, in shock, or hypotensive. The owner triage summary at the beginning of the article emphasizes this point, and it cannot be overstated. Before administering robenacoxib, the veterinarian should assess the cat's hydration status, which can be done through physical examination (skin turgor, mucous membrane moisture) and laboratory testing (packed cell volume, total protein). If there is any doubt about hydration status, intravenous or subcutaneous fluids should be administered before the NSAID. The combination of robenacoxib with other drugs that affect renal function, such as diuretics and ACE inhibitors, requires careful consideration. The study combining robenacoxib with benazepril and furosemide in healthy cats found the combination was well tolerated [9]. However, this was in healthy cats with normal renal function. In a cat with CKD, the combination of an NSAID, an ACE inhibitor, and a diuretic can lead to a dangerous drop in glomerular filtration rate. If this combination is necessary, it should be used with close monitoring of renal parameters, and the owner should be instructed to watch for signs of decreased urine output or lethargy. ## Hepatic Considerations and Drug Metabolism While renal toxicity is the most feared complication of NSAID use, hepatic effects are also possible. Robenacoxib is metabolized in the liver, and cats with hepatic insufficiency may have reduced drug clearance, leading to higher blood concentrations and an increased risk of adverse effects. The available data on robenacoxib use in cats with liver disease is limited. The drug should be used with caution in cats with elevated liver enzymes or clinical signs of hepatic disease, and liver function should be monitored during treatment. The short blood half-life of robenacoxib, approximately 1.1 to 1.5 hours, is a favorable feature in the context of hepatic disease [5][2]. Because the drug is cleared relatively quickly from the bloodstream, any accumulation due to reduced hepatic metabolism is less likely to be clinically significant than with drugs that have a longer half-life. However, the drug's concentration in inflamed tissue, which is the site of action, may be less affected by hepatic clearance. This means that the anti-inflammatory effect may be preserved even in cats with hepatic insufficiency, but the risk of systemic toxicity may be increased. Clinical signs of hepatotoxicity include vomiting, anorexia, lethargy, and jaundice. If any of these signs develop during robenacoxib therapy, the drug should be discontinued immediately, and a serum biochemistry profile should be performed to evaluate liver enzymes and bilirubin. Supportive care, including intravenous fluids and hepatoprotectants, may be necessary. The prognosis for drug-induced hepatotoxicity is generally good if the drug is discontinued early, but it can be fatal in severe cases. ## The Role of Robenacoxib in a Multimodal Pain Management Plan Effective pain management in cats often requires a multimodal approach, combining drugs with different mechanisms of action to achieve better analgesia with lower doses of each drug. Robenacoxib is an excellent foundation for this approach, but it should not be the only tool in the toolbox. For severe acute pain, such as that associated with major orthopedic surgery, robenacoxib may be combined with opioids, such as buprenorphine or methadone. Opioids provide rapid, potent analgesia by acting on opioid receptors in the central nervous system, while robenacoxib provides longer-lasting anti-inflammatory effects by inhibiting COX-2. This combination allows for lower doses of each drug, reducing the risk of side effects. For chronic pain, robenacoxib may be combined with non-pharmacological therapies, such as weight management, physical therapy, and environmental modification. It may also be combined with other analgesics, such as gabapentin, which is often used for neuropathic pain and as an adjunct for chronic musculoskeletal pain. Gabapentin has a different mechanism of action, modulating calcium channels in the central nervous system, and can be used in combination with NSAIDs. The combination of robenacoxib and gabapentin is commonly used in clinical practice, although there is limited published evidence specifically evaluating this combination in cats. It is important to emphasize that robenacoxib should never be combined with other NSAIDs or corticosteroids. The concurrent use of two NSAIDs or an NSAID and a corticosteroid dramatically increases the risk of gastrointestinal ulceration, renal damage, and other serious adverse effects. This is a strict contraindication. If a cat is already receiving another NSAID or a corticosteroid, a washout period of at least 5 to 7 days is recommended before starting robenacoxib, although the exact duration of the washout period depends on the drugs involved and the individual patient. ## Prognosis and Long-Term Monitoring The prognosis for cats receiving robenacoxib is generally favorable. For acute conditions, such as postoperative pain, the prognosis is excellent, and most cats recover fully with appropriate pain management. For chronic conditions, such as osteoarthritis, the prognosis is more variable. Robenacoxib can significantly improve quality of life, but it is not a cure. Osteoarthritis is a progressive disease, and the cat's condition may worsen over time despite treatment. Regular reassessment is essential to adjust the treatment plan as the disease progresses. Long-term monitoring for cats on chronic robenacoxib therapy should include a physical examination every 3 to 6 months, with particular attention to body condition score, muscle mass, and joint mobility. A serum biochemistry profile, including renal and hepatic parameters, should be performed at least every 6 months, and more frequently in cats with pre-existing disease. Blood pressure should be measured regularly, as hypertension can develop or worsen in cats on NSAID therapy. The owner should be asked about the cat's appetite, activity level, and any signs of gastrointestinal upset. The dose of robenacoxib should be reassessed at each visit. The goal is to use the lowest effective dose for the shortest duration necessary. For some cats, it may be possible to reduce the dose or administer the drug on an alternating-day basis. For others, the dose may need to be increased or a different analgesic added. The decision should be based on the cat's clinical response and any adverse effects observed. ## Special Populations: Geriatric, Pediatric, and Pregnant or Lactating Cats Geriatric cats are the most common candidates for robenacoxib therapy, given the high prevalence of osteoarthritis in this population. However, they are also the most vulnerable to adverse effects. Age-related changes in renal and hepatic function can affect drug clearance, and the presence of concurrent diseases increases the risk of drug interactions. In geriatric cats, it is particularly important to perform a thorough diagnostic workup before starting robenacoxib and to monitor closely during treatment. A lower starting dose, at the lower end of the approved range (1 mg/kg), may be considered, with dose escalation based on clinical response and tolerance. Pediatric cats, particularly kittens under 6 months of age, have not been extensively studied in clinical trials of robenacoxib. The safety and efficacy of the drug in this population are not well established. The drug is not approved for use in kittens, and its use should be avoided unless the potential benefits clearly outweigh the risks. If robenacoxib is used in a kitten, it should be at the lowest effective dose and for the shortest duration possible, with close monitoring. The use of robenacoxib in pregnant or lactating cats is contraindicated. NSAIDs can affect fetal development, particularly the cardiovascular system, and can cause premature closure of the ductus arteriosus. They can also affect uterine contractions and prolong labor. In lactating cats, NSAIDs can be excreted in milk and may affect the nursing kittens. If a pregnant or lactating cat requires pain management, alternative analgesics, such as opioids, should be considered. ## Owner Education and Compliance The success of robenacoxib therapy depends heavily on owner education and compliance. Owners need to understand why the drug has been prescribed, how to administer it correctly, and what to watch for. The veterinarian or veterinary technician should take the time to explain the dosing schedule, the importance of giving the drug on an empty stomach, and the potential side effects. Written instructions should be provided, and the owner should be encouraged to call with any questions or concerns. One common issue is the difficulty of administering oral medication to cats. Onsior tablets are flavored, which can help with palatability, but some cats will still resist. Owners can try hiding the tablet in a small amount of a highly palatable treat, such as tuna or a commercial pill pocket. Alternatively, the tablet can be placed directly into the cat's mouth, followed by a small amount of water or a treat to ensure it is swallowed. The tablet should not be crushed or split, as this can affect drug stability and dose accuracy. Owners should also be educated about the dangers of giving human NSAIDs to cats. Ibuprofen, naproxen, and aspirin are extremely toxic to cats and can cause severe gastrointestinal ulcers, kidney failure, and death. Acetaminophen (paracetamol) is also highly toxic and can cause methemoglobinemia and liver failure. These drugs should never be given to a cat, even in an emergency. If a cat accidentally ingests a human NSAID, immediate veterinary care is essential. ## Conclusion and Clinical Takeaway Robenacoxib is a valuable tool in the feline pain management armamentarium. Its high COX-2 selectivity, short blood half-life, and tissue-specific accumulation provide a favorable safety profile compared to non-selective NSAIDs. The approved dosing range of 1 to 2.4 mg/kg orally once daily, or 2 mg/kg subcutaneously for perioperative use, provides flexibility for the veterinarian to tailor therapy to the individual patient [2][3]. Clinical trials have demonstrated efficacy for both acute postoperative pain and chronic osteoarthritis-associated pain, with a safety profile comparable to placebo in most studies [15][4][3]. However, robenacoxib is not without risks. The most significant concern is renal toxicity, particularly in cats with pre-existing kidney disease, dehydration, or hypotension. A thorough diagnostic workup before treatment, careful patient selection, and regular monitoring during treatment are essential to minimize these risks. The drug is contraindicated in dehydrated or hypotensive cats and should be used with caution in cats with significant renal or hepatic impairment. Owner education is a critical component of safe robenacoxib use. Owners must understand the dosing schedule, the importance of administering the drug on an empty stomach, and the signs of adverse effects that warrant immediate veterinary attention. They must also be warned about the dangers of human NSAIDs and the importance of never combining robenacoxib with other NSAIDs or corticosteroids. In summary, robenacoxib is a safe and effective NSAID for cats when used appropriately. The key to successful therapy lies in careful patient selection, accurate dosing, close monitoring, and thorough owner education. By following the evidence-based protocol outlined in this article, veterinarians can provide effective pain relief for their feline patients while minimizing the risk of adverse effects. ## Frequently Asked Questions ### 1. What is the exact dose of Onsior for my cat? The approved dose of robenacoxib for cats is 1.0 to 2.4 mg/kg orally once daily, or 2 mg/kg subcutaneously for injection [2][3]. Your veterinarian will calculate the precise dose based on your cat's body weight and the condition being treated. Do not adjust the dose without veterinary guidance. ### 2. Can I give my cat Onsior with food? It is recommended to give Onsior on an empty stomach, either one hour before or two hours after a meal, because food significantly reduces its absorption [6]. However, if your cat vomits on an empty stomach, you can give it with a small amount of food, though efficacy may be slightly reduced. ### 3. How long does it take for Onsior to work in cats? Robenacoxib reaches peak blood concentrations within 1 hour of oral administration [6]. However, its full analgesic effect may take a few hours to become apparent. For acute postoperative pain, the injectable form provides faster relief. ### 4. Is Onsior safe for cats with kidney disease? Studies have shown that robenacoxib is well tolerated in cats with stable chronic kidney disease, with no significant increase in adverse events compared to placebo [8][4]. However, it should be used with caution and under close veterinary supervision, with monitoring of kidney function. It is contraindicated in dehydrated or hypotensive cats. ### 5. Can I give Onsior with other pain medications? No. Onsior should never be combined with other NSAIDs or corticosteroids, as this increases the risk of severe side effects. If additional pain relief is needed, your veterinarian may prescribe an opioid or other non-NSAID analgesic. ### 6. What happens if I miss a dose of Onsior? If you miss a dose, give it as soon as you remember, but skip it if it is almost time for the next dose. Never give a double dose to make up for a missed one. Resume the normal dosing schedule the next day. ### 7. How long can a cat stay on Onsior? For chronic conditions like osteoarthritis, robenacoxib has been studied for up to 12 weeks and has been shown to be safe [4]. For acute conditions, it is typically used for a few days. The duration of treatment should be determined by your veterinarian based on your cat's response and any underlying health issues. ### 8. What are the most common side effects of Onsior in cats? The most common side effects reported are gastrointestinal, such as vomiting, diarrhea, and reduced appetite. In clinical trials, the incidence of these events was similar to that seen with placebo [8][4]. More serious side effects, such as kidney or liver issues, are rare but possible, which is why monitoring is important. **Veterinary Disclaimer:** This article is educational and is not a substitute for veterinary diagnosis or treatment. Always consult with a qualified veterinarian regarding the health and treatment of your pet. ## Sources [1] [Robenacoxib in the treatment of pain in cats and dogs: safety, efficacy, and place in therapy.](https://pubmed.ncbi.nlm.nih.gov/30148083/) [2] [Pharmacology, safety, efficacy and clinical uses of the COX-2 inhibitor robenacoxib.](https://pubmed.ncbi.nlm.nih.gov/35460083/) [3] [Evaluation of injectable robenacoxib for the treatment of post-operative pain in cats: results of a randomized, masked, placebo-controlled clinical trial.](https://pubmed.ncbi.nlm.nih.gov/27686251/) [4] [Clinical safety of robenacoxib in cats with chronic musculoskeletal disease.](https://pubmed.ncbi.nlm.nih.gov/34196973/) [5] [Differential pharmacokinetics and pharmacokinetic/pharmacodynamic modelling of robenacoxib and ketoprofen in a feline model of inflammation.](https://pubmed.ncbi.nlm.nih.gov/24628410/) [6] [Effects of route of administration and feeding schedule on pharmacokinetics of robenacoxib in cats.](https://pubmed.ncbi.nlm.nih.gov/23438125/) [7] [Determination of the route of excretion of robenacoxib (Onsior™) in cats and dogs: A pilot study.](https://pubmed.ncbi.nlm.nih.gov/33881783/) [8] [Clinical safety of robenacoxib in feline osteoarthritis: results of a randomized, blinded, placebo-controlled clinical trial.](https://pubmed.ncbi.nlm.nih.gov/26058587/) [9] [Effect of benazepril, robenacoxib and their combination on glomerular filtration rate in cats.](https://pubmed.ncbi.nlm.nih.gov/27338786/) [10] [Modeling of Large Pharmacokinetic Data Using Nonlinear Mixed-Effects: A Paradigm Shift in Veterinary Pharmacology. A Case Study With Robenacoxib in Cats.](https://pubmed.ncbi.nlm.nih.gov/27770596/) [11] [Effect of meloxicam or robenacoxib administration timing on renal function and postoperative analgesia in cats undergoing ovariohysterectomy: A randomized, blinded, controlled clinical trial.](https://pubmed.ncbi.nlm.nih.gov/38235901/) [12] [Robenacoxib versus meloxicam for the control of peri-operative pain and inflammation associated with orthopaedic surgery in cats: a randomised clinical trial.](https://pubmed.ncbi.nlm.nih.gov/25880535/) [13] [Safety evaluation of the interchangeable use of robenacoxib in commercially-available tablets and solution for injection in cats.](https://pubmed.ncbi.nlm.nih.gov/32988403/) [14] [Evaluation of cats treated with robenacoxib after gastrointestinal surgery.](https://pubmed.ncbi.nlm.nih.gov/39540680/) [15] [Robenacoxib shows efficacy for the treatment of chronic degenerative joint disease-associated pain in cats: a randomized and blinded pilot clinical trial.](https://pubmed.ncbi.nlm.nih.gov/33833276/) [16] [Evaluation of the analgesic efficacy of grapiprant compared with robenacoxib in cats undergoing elective ovariohysterectomy in a prospective, randomized, masked, non-inferiority clinical trial.](https://pubmed.ncbi.nlm.nih.gov/38511293/) [17] [Comparison of post-operative analgesic efficacy of tolfenamic acid and robenacoxib in ovariohysterectomized cats.](https://pubmed.ncbi.nlm.nih.gov/29695672/) [18] [Effects of selective cyclooxygenase-2 inhibitor robenacoxib on primary cells derived from feline injection-site sarcoma.](https://pubmed.ncbi.nlm.nih.gov/37334757/) [19] [Effect of oral administration of robenacoxib on inhibition of paracentesis-induced blood-aqueous barrier breakdown in healthy cats.](https://pubmed.ncbi.nlm.nih.gov/29583043/) [20] [Robenacoxib versus meloxicam following ovariohysterectomy in cats: A randomised, prospective clinical trial involving owner-based assessment of pain.](https://pubmed.ncbi.nlm.nih.gov/37494365/) ## Related Clinical & Scientific Guides * [Drontal Plus Dewormer: Broad-Spectrum Dose Guide by Weight](/knowledge/veterinary-medicine/calculators-clinical-tools/drontal-plus-dewormer-broad-spectrum-dose-guide-by-weight) * [Panacur Fenbendazole Dewormer: Dog Fecal Parasite Treatment Schedule](/knowledge/veterinary-medicine/calculators-clinical-tools/panacur-fenbendazole-dewormer-dog-fecal-parasite-treatment-schedule) * [Prednisolone for Cats: Feline Safe Steroid Dose and Autoimmune](/knowledge/veterinary-medicine/calculators-clinical-tools/prednisolone-for-cats-feline-safe-steroid-dose-and-autoimmune)