# Phenobarbital for Dogs: Side Effects and Overdose Signs Phenobarbital for dogs side effects are usually mild at first, most often sleepiness, wobbliness, and increased appetite. Some dogs develop serious problems over time, including liver injury and blood cell changes. Overdose can cause dangerous sedation, trouble breathing, and loss of consciousness. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - Mild sedation, unsteadiness, and hunger are the most common early effects, and many dogs adjust within a few weeks. - Serious effects include liver damage, low blood cell counts, and skin sores, so routine blood work matters. - Overdose signs include extreme drowsiness, staggering, vomiting, slow breathing, and collapse. Call for emergency care right away. - Never change or stop the dose on your own. Stopping suddenly can trigger seizures. - Drug interactions are real. Tell your veterinarian about every medication, supplement, or antifungal your dog takes [1]. ## What Phenobarbital Is and What Veterinarians Use It For Phenobarbital (often shortened to PB) is an antiseizure medication. It is the first-line treatment for canine epilepsy because it works well, costs less than many alternatives, and stays in the body in a predictable way [2]. Veterinarians prescribe it most often to control seizures in dogs with epilepsy. It can also be used for other conditions where seizure control or sedation helps. In one reported case, a dog with a rare salivary gland condition called phenobarbital-responsive sialadenosis improved markedly once oral phenobarbital was started [3]. That is an uncommon use, but it shows how the drug can help beyond classic epilepsy. Phenobarbital is sometimes combined with other antiseizure medications. Levetiracetam is a common add-on because it is cleared mostly by the kidneys and has limited liver metabolism [4]. Gabapentin is another drug that may be added when phenobarbital alone does not fully control signs [3]. ## How Phenobarbital Works in Dogs Phenobarbital calms excessive electrical activity in the brain. It raises the seizure threshold, which makes it harder for a seizure to start and spread. The drug is processed mainly by the liver through a family of enzymes called cytochrome P450 [1][5]. In dogs, several of these enzymes are involved in breaking phenobarbital down, including forms in the 2C and 3A families [5]. Because the liver does this work, anything that affects those enzymes can change how much drug stays in your dog's bloodstream. That is why phenobarbital has what veterinarians call a narrow therapeutic index. The gap between a helpful dose and a harmful one is small, and dogs vary a lot in how they handle the drug [2]. Two dogs on the same dose can end up with very different blood levels. ## Dosing Basics: Who Decides, How It Is Given, and Missed Doses Your veterinarian decides the dose. It is calculated from your dog's body weight and health, and it may be adjusted over time based on blood tests. Never borrow a dose from another dog or change it yourself. Phenobarbital is usually given by mouth, often twice a day. One published case used 3.6 mg/kg given orally twice daily for a dog with sialadenosis [3]. Another study used 2.5 mg/kg every 12 hours in dogs with distemper [6]. These are examples from specific cases, not a general recommendation. Your dog's dose will be different. Give the medication at the same times each day. Steady timing keeps the drug level stable in the blood. Most dogs take tablets or capsules well, and some formulations can be hidden in a small treat if your veterinarian approves. If you miss a dose, do not double up. Call your veterinarian or your pharmacy for guidance on that specific situation. If you miss several doses in a row, contact your veterinarian the same day. Sudden gaps in treatment can let seizures break through. ## Phenobarbital for Dogs Side Effects: What to Watch For Most owners notice some change in the first days to weeks. Many of these effects ease as the body adjusts. Others need a phone call or a visit. | Side effect | How common or serious | What to do | |---|---|---| | Sleepiness and sedation | Very common, usually mild to moderate | Give the drug as prescribed and note the pattern. Call your vet if your dog is hard to wake. | | Wobbliness or unsteady walking (ataxia) | Common, especially early or after a dose increase | Keep your dog away from stairs and slick floors. Call your vet if it does not improve or gets worse. | | Increased appetite and weight gain | Common | Measure food and treats. Ask your vet about a weight plan. | | Increased thirst and urination | Common | Keep fresh water available. Mention any major change at the next visit. | | Vomiting or poor appetite | Less common | Call your vet if it lasts more than a day or your dog stops eating. | | Liver injury | Serious, can develop over months | Routine blood work catches it early. Report yellow gums, vomiting, or lethargy right away. | | Low blood cell counts | Serious but uncommon | Watch for pale gums, bruising, or unusual tiredness. Seek care promptly. | | Skin sores or rashes | Uncommon | Have your vet look at any new sore, especially on the belly or paws. | | Behavior changes, restlessness, or anxiety | Possible | Discuss with your vet. Do not stop the drug on your own. | Some owners ask about phenobarbital for dogs anxiety. Phenobarbital is not an anxiety medication. It is a seizure drug that causes sedation as a side effect. If your dog has anxiety, your veterinarian will likely recommend a different treatment. If your dog takes anxiety medication and phenobarbital together, ask your vet about interactions, and know the warning signs covered in [Dog Anxiety Medication Overdose: Signs and Emergency Steps](/knowledge/veterinary-medicine/anxiety-and-behavior-support/dog-anxiety-medication-overdose). Sedation and ataxia are the classic signs of too much phenobarbital in the blood. In a 2025 report, two epileptic dogs on long-term phenobarbital became markedly sedated and unsteady after they were also given fluconazole, an antifungal drug. Their phenobarbital blood levels rose, and the signs resolved after the antifungal was stopped [1]. That case shows how a second medication can push a stable dog into toxicity. ## Who Should Not Take It and What Interacts With It Phenobarbital is not right for every dog. Your veterinarian will weigh the risks if your dog has liver disease, kidney disease, or certain blood disorders. Puppies, pregnant dogs, and dogs with other serious illnesses need careful evaluation. Drug interactions are a major concern. Fluconazole inhibits the liver enzymes that break down phenobarbital, which can raise blood levels and cause sedation and ataxia [1]. This interaction happened even in dogs with a specific genetic deletion in a cytochrome P450 gene, which suggests the interaction does not depend on that one enzyme [1]. Other medications can interact too. Any drug that affects liver enzymes, the nervous system, or blood clotting may change how phenobarbital behaves. Tell your veterinarian about everything your dog takes, including: - Other seizure medications - Antifungals such as fluconazole - Antibiotics - Pain relievers, including over-the-counter products - Supplements and herbal products - Anxiety or behavior medications If your dog takes another medication and starts acting sedated or unsteady, call your veterinarian. Do not wait to see if it passes. Monitoring phenobarbital blood levels during certain drug combinations is advised [1]. ## Phenobarbital for Dogs Overdose: Signs and Emergency Steps An overdose can happen from a dosing mistake, a dog eating the pill bottle, or a drug interaction that raises blood levels [1]. The signs can escalate quickly. Watch for these overdose signs: - Extreme drowsiness or difficulty waking your dog - Staggering, falling, or inability to stand - Vomiting, drooling, or retching - Slow, shallow, or irregular breathing - Low body temperature - Weak pulse or pale gums - Collapse, coma, or seizures that will not stop If you see any of these, treat it as an emergency. Call your veterinarian or an emergency animal hospital immediately. If your regular clinic is closed, go to the nearest 24-hour emergency facility. Bring the pill bottle with you so the team knows the strength and how many pills may be missing. You can also call poison control for animals. Have your dog's weight, the medication strength, and the time of exposure ready. Do not try to make your dog vomit unless a veterinarian tells you to. If your dog overdoses on any medication, the steps in [What to Do If Your Dog Overdoses on Medication: Emergency Steps](/knowledge/veterinary-medicine/pet-medication-guide/dog-medication-overdose-emergency-steps) can help you act fast while you are on the way to care. Treatment for phenobarbital overdose depends on how much was taken and how your dog is doing. Your veterinarian may give fluids, activated charcoal, or other supportive care. Some dogs need to stay in the hospital for monitoring. ## Limitations and When to Contact a Veterinarian This article covers common patterns, but every dog is different. Your dog's dose, blood levels, and health history all affect what is safe. Individual cases need a veterinarian who knows your dog. Call your veterinarian the same day if you notice: - New or worsening wobbliness - Vomiting or refusal to eat for more than 24 hours - Yellow gums, eyes, or skin - Pale gums or easy bruising - A new skin sore or rash - Sedation that seems deeper than usual - Any missed doses Go to an emergency clinic right away if you see: - Your dog cannot be woken - Breathing is slow, shallow, or labored - Your dog collapses or has a seizure that will not stop - You know or suspect a large overdose Keep routine visits on schedule. Blood work to check liver values and drug levels is a normal part of phenobarbital treatment [2]. These visits catch problems before they become emergencies. If your dog also takes other medications, ask your veterinarian whether any of them need monitoring too. Long-term medication use can have effects that build slowly, similar to what owners ask about with [What Are the Long-Term Side Effects of Apoquel for Dogs?](/knowledge/veterinary-medicine/pet-medication-guide/apoquel-long-term-side-effects-dogs). ## Frequently Asked Questions ### Will my dog be sleepy forever on phenobarbital? Most dogs adjust to the sedation within a few weeks. Some remain a bit calmer than before, which many owners accept as a trade-off for seizure control. If your dog stays deeply sedated or gets worse over time, call your veterinarian. ### Can I stop phenobarbital if my dog seems fine? No. Stopping suddenly can cause seizures to return, sometimes worse than before. Only your veterinarian should change or taper the dose. If cost or side effects are a concern, talk to your vet about options. ### Does phenobarbital help with anxiety in dogs? Phenobarbital is not an anxiety medication. It is used for seizures, and sedation is a side effect, not a treatment goal. If your dog has anxiety, ask your veterinarian about medications designed for behavior problems. ### How often does my dog need blood tests? Your veterinarian will set the schedule, often starting a few weeks after treatment begins and then at regular intervals. Testing checks the drug level and looks at liver and blood values [2]. More frequent testing may be needed after dose changes or new medications [1]. ### What should I do if I miss a dose? Call your veterinarian or pharmacy for advice on that specific situation. Do not give a double dose to make up for a missed one. If you miss several doses, contact your veterinarian the same day, because seizure control can slip. ## Related Articles - [Dog Anxiety Medication Overdose: Signs and Emergency Steps](/knowledge/veterinary-medicine/anxiety-and-behavior-support/dog-anxiety-medication-overdose) - [What Are the Side Effects of Trazodone for Dogs?](/knowledge/veterinary-medicine/pet-medication-guide/trazodone-side-effects-dogs) - [What to Do If Your Dog Overdoses on Medication: Emergency Steps](/knowledge/veterinary-medicine/pet-medication-guide/dog-medication-overdose-emergency-steps) - [What Are the Long-Term Side Effects of Apoquel for Dogs?](/knowledge/veterinary-medicine/pet-medication-guide/apoquel-long-term-side-effects-dogs) - [Dog Diabetes: How to Tell If Your Dog's Insulin Dose Is Too High](/knowledge/veterinary-medicine/senior-and-chronic-care/dog-diabetes-insulin-overdose-signs) - [What Are the Side Effects of Gabapentin for Dogs?](/knowledge/veterinary-medicine/pet-medication-guide/gabapentin-side-effects-dogs) - [Phenobarbital and Liver Failure in Dogs: What Owners Should Know](/knowledge/veterinary-medicine/pet-medication-guide/phenobarbital-liver-failure-dogs) - [Phenobarbital for Dogs: With or Without Food?](/knowledge/veterinary-medicine/pet-medication-guide/phenobarbital-dogs-with-without-food) - [Phenobarbital vs Keppra for Dogs: Which Seizure Drug?](/knowledge/veterinary-medicine/pet-medication-guide/phenobarbital-vs-keppra-dogs) ## References 1. [Adverse Drug-Drug Interaction Between Phenobarbital and Fluconazole in Two Dogs](https://pubmed.ncbi.nlm.nih.gov/40682369/). Journal of veterinary internal medicine, 2025. 2. [Pharmacometrics modeling and simulation to assist phenobarbital dose optimization in dogs](https://pubmed.ncbi.nlm.nih.gov/40969351/). Frontiers in veterinary science, 2025. 3. [Effectiveness of gabapentin in combination therapy for phenobarbital-responsive sialadenosis in a Toy Poodle](https://pubmed.ncbi.nlm.nih.gov/41371661/). The Journal of veterinary medical science, 2026. 4. [Short-Term Renal Effects of Add-On Levetiracetam in Client-Owned Dogs with Idiopathic Epilepsy Receiving Phenobarbital: A Four-Week Prospective Study](https://pubmed.ncbi.nlm.nih.gov/42797888/). Veterinary sciences, 2026. 5. [Different Roles of Canine Cytochrome P450 2C and 3A Enzymes Involved in the Oxidative Metabolism of Phenobarbital, a Key Anticonvulsant for Dogs: Comparison With Human P450 Enzymes](https://pubmed.ncbi.nlm.nih.gov/42410958/). Journal of veterinary pharmacology and therapeutics, 2026. 6. [Effects of Phenobarbital and Prednisolone on Neurological Signs of Canine Distemper](https://pubmed.ncbi.nlm.nih.gov/40833346/). Veterinary medicine and science, 2025. ## Further Reading - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Dog Owners](https://www.merckvetmanual.com/dog-owners) - [Cornell Richard P. Riney Canine Health Center: Canine Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information)