# Zyrtec and Diarrhea in Cats: What Owners Should Know Cat Zyrtec diarrhea is a question many owners ask after giving their cat cetirizine, the generic name for Zyrtec, or after wondering whether they can. Zyrtec is sometimes used in cats for allergy and itch relief, but it is not approved for cats by the FDA, and diarrhea is one of the digestive side effects owners report. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - Zyrtec (cetirizine) is a human antihistamine that veterinarians sometimes prescribe off label for cats with allergies or itching, but it is not FDA approved for cats. - Diarrhea can happen after Zyrtec, though it is not the most common reaction. Any new medicine can upset a cat's stomach and change stool consistency. - Never give your cat a whole human tablet without a veterinarian's direction. The dose depends on your cat's weight and health, and your veterinarian calculates it. - If diarrhea starts after a Zyrtec dose, stop the medicine and call your veterinarian. Bring the package so they know the strength. - Seek emergency care right away for repeated vomiting, blood in stool, weakness, trouble breathing, or a swollen face. ## What Is Zyrtec and Why Would a Cat Take It? Zyrtec is the brand name for cetirizine, a second generation antihistamine sold for people. It blocks histamine, the chemical the body releases during allergic reactions. In people it treats hay fever, hives, and itchy skin. Cats get allergies too. They can react to pollen, dust, mold, flea saliva, and some foods. The itch often shows up as scratching, chewing, hair loss, and miliary dermatitis, which is a bumpy rash along the back and neck. Veterinarians may reach for an antihistamine when the itch is mild or when steroids are not a good option. Zyrtec is used off label in cats. Off label means the drug is legal and available, but it has not gone through the FDA approval process for that species. Your veterinarian can still prescribe it if they believe the benefits outweigh the risks for your cat. Antihistamines like cetirizine are not the first choice for every itchy cat. Response varies, and some cats get little relief. Your veterinarian may combine it with other treatments or switch to a different drug. ## How Zyrtec Works in a Cat After your cat swallows a dose, cetirizine is absorbed from the gut into the bloodstream. It travels to tissues and blocks H1 histamine receptors. When histamine cannot dock at those receptors, the itching, sneezing, and runny eyes tied to histamine are reduced. Cetirizine is considered less sedating than older antihistamines such as diphenhydramine. That said, individual cats vary. Some become sleepy, some stay alert, and a few become restless. The drug is cleared mainly through the kidneys. That matters for cats with kidney disease, because a drug that leaves the body slowly can build up. Your veterinarian will factor kidney and liver health into the decision. ## Dosing Basics for Cats Your veterinarian decides the dose. They calculate it from your cat's weight, overall health, kidney and liver function, and what else your cat takes. Do not copy a dose from a friend's cat or from an internet chart. Cats differ a lot in size and in how they handle drugs. Zyrtec comes as tablets and as a liquid. Many human liquid versions contain sweeteners, alcohol, or other additives that may not suit cats. Plain cetirizine tablets are often easier to split accurately. Ask your veterinarian which form to use and how to measure it. Give the medicine by mouth, with or without food unless your veterinarian says otherwise. Some cats do better with a small treat or a bit of food to settle the stomach. If your cat spits the pill out, do not re-dose blindly. You may not know how much was swallowed. If you miss a dose, do not double up. Give the next dose at the regular time, or call your veterinarian for guidance. Doubling a dose raises the chance of side effects, including stomach upset and diarrhea. Never crush or hide a pill in a whole meal unless your veterinarian approves. If your cat eats only part of the food, the dose is uncertain. ## Zyrtec for Cats Side Effects: Common and Serious Most cats tolerate cetirizine, but reactions happen. Diarrhea is one of them. When a cat's large intestine is irritated by illness, parasites, or other causes, diarrhea is often the result [1]. A new drug can act as that irritant in a sensitive cat. Here is a practical breakdown. | Side effect | How common or serious | What to do | |---|---|---| | Sleepiness or mild sedation | Common, usually mild | Watch your cat. Call your vet if it lasts more than a day or seems heavy | | Diarrhea, soft stool | Uncommon to occasional, usually mild | Stop the drug and call your vet. See the diarrhea guide below | | Vomiting | Uncommon | Stop the drug. Call your vet if it repeats or your cat stops eating | | Drooling or lip smacking after a pill | Common, not dangerous | Offer water. Try a treat after the next dose if your vet agrees | | Loss of appetite | Uncommon | Call your vet if your cat skips more than one meal | | Restlessness or agitation | Uncommon | Stop the drug and call your vet | | Fast heart rate, tremors, seizures | Rare, serious | Emergency care now | | Trouble breathing, facial swelling, hives | Rare, serious allergic reaction | Emergency care now | Diarrhea in cats is common and has many causes. In one study of cats in multi cat households, potential enteropathogenic bacteria were found in some cats with diarrhea, though the link was not simple [2]. Other work shows that gut bacteria from diarrheic cats can impair the intestinal barrier and trigger colon inflammation in recipient animals [3]. Chronic diarrhea in cats has many underlying conditions, and the mechanisms include osmotic and secretory diarrhea, poor barrier function, and abnormal gut motility [4]. Feline coronavirus and Clostridium perfringens toxin are also found often in cats with chronic diarrhea [5]. Intestinal parasites such as Cystoisospora, Giardia, and Toxocara appear in cats with diarrhea as well [6]. That long list is the point. A loose stool after a Zyrtec dose may be drug related, or it may be a coincidence that lines up with a parasite, a diet change, or an infection. Your veterinarian sorts this out. If the diarrhea started right after a new medicine, read [What to Do If Your Cat Has Diarrhea After Starting a New Medication](/knowledge/veterinary-medicine/pet-medication-guide/cat-diarrhea-after-new-medication). If you already know a medicine is the likely trigger, see [What to Do If Your Cat's Medication Is Causing Diarrhea](/knowledge/veterinary-medicine/pet-medication-guide/cat-medication-diarrhea-what-to-do). ## Who Should Not Take Zyrtec Some cats should not get cetirizine, or should get it only with close monitoring. - Cats with kidney disease. The drug is cleared by the kidneys, so dosing may need to change or the drug may be a poor fit. - Cats with liver disease. Liver problems can change how drugs are processed. - Pregnant or nursing cats. Safety is not well established in veterinary research. - Kittens. Safety and dosing in very young cats are not well established. - Cats with a known allergy to cetirizine or similar antihistamines. - Cats already on sedatives, opioids, or other antihistamines. The combined effect can be strong sedation. Drug interactions matter. Tell your veterinarian about everything your cat takes, including prescriptions, over the counter products, supplements, and flea or tick products. Cetirizine can add to the drowsiness caused by other sedating drugs. Your veterinarian may adjust doses or choose a different treatment. Do not give human combination products. Many cold and allergy products mix cetirizine with pseudoephedrine, phenylephrine, or acetaminophen. Acetaminophen is toxic to cats. Always check the label for a single active ingredient, and confirm with your veterinarian before giving anything. ## Overdose and Poison Control An overdose can cause heavy sedation, agitation, fast heart rate, tremors, vomiting, and diarrhea. The risk rises if your cat ate several tablets or drank from a liquid bottle. If you think your cat took too much Zyrtec, call your veterinarian or an animal poison control hotline right away. Have the product bottle, the strength, and your best estimate of how much your cat swallowed. Do not wait for symptoms to appear. If your cat is collapsing, having seizures, breathing hard, or cannot stand, go to an emergency clinic now. ## Limitations and When to Contact a Veterinarian This article cannot tell you whether your cat's diarrhea is from Zyrtec or from something else. Individual cases need a veterinarian who can examine your cat, ask about the timeline, and run tests if needed. Call your veterinarian the same day if: - Diarrhea starts within a day or two of a Zyrtec dose - Stool is soft for more than 24 hours or watery for more than 12 hours - Your cat vomits more than once - Your cat eats less or seems dull - You are unsure how much medicine your cat swallowed Go to an emergency clinic now if: - There is blood or black tarry material in the stool - Your cat is weak, wobbly, or cannot stand - Breathing is fast, noisy, or labored - The face, muzzle, or paws swell - There is a seizure, tremor, or collapse - Your cat is straining to poop with nothing coming out Schedule a routine visit if the diarrhea comes and goes over weeks, if your cat is losing weight, or if the stool has mucus or a greasy look. Chronic diarrhea needs a real diagnosis. Your veterinarian may ask detailed questions, do a physical exam, try a specific food trial, and run blood, urine, or fecal tests, plus x-rays, ultrasound, endoscopy, or biopsies if the picture is unclear [1]. For home care basics while you wait for the appointment, see [Cat Diarrhea Treatment](/knowledge/veterinary-medicine/symptom-guides/cat-diarrhea-treatment) and [Cat Diarrhea: Causes, Home Care, and When to Worry](/knowledge/veterinary-medicine/online-vet-and-care-costs/cat-diarrhea-causes-home-care). If vomiting is part of the picture, read [Cat Vomiting and Diarrhea: When to Worry](/knowledge/veterinary-medicine/digestive-and-urinary-care/cat-vomiting-diarrhea-when-to-worry). ## Frequently Asked Questions ### Can cats take Zyrtec? Yes, cats can take Zyrtec when a veterinarian prescribes it off label, but it is not FDA approved for cats. Your veterinarian sets the dose based on your cat's weight and health. Never give it on your own. ### Is diarrhea a common Zyrtec side effect in cats? Diarrhea is possible but not the most typical reaction. Sleepiness and drooling are seen more often. Any new medicine can upset a cat's stomach, so a loose stool after a dose deserves a call to your veterinarian. ### How long does cetirizine diarrhea last in cats? Mild drug related diarrhea often settles within a day or two after the medicine stops. If it lasts longer, gets worse, or comes with vomiting or appetite loss, your cat needs to be seen. Do not wait it out for more than a day. ### What should I do if my cat has diarrhea after Zyrtec? Stop the medicine and call your veterinarian. Offer water so your cat stays hydrated, and note when the diarrhea started and what the stool looks like. Do not give a second dose or any human anti diarrhea product without veterinary direction. ### Can I give my cat a human Zyrtec tablet? Only if your veterinarian tells you to, with the exact strength and amount they choose. Many human products mix cetirizine with other drugs that are unsafe for cats. Check the label and confirm with your veterinarian first. ## Related Articles - [What to Do If Your Cat Has Diarrhea After Starting a New Medication](/knowledge/veterinary-medicine/pet-medication-guide/cat-diarrhea-after-new-medication) - [What to Do If Your Cat's Medication Is Causing Diarrhea](/knowledge/veterinary-medicine/pet-medication-guide/cat-medication-diarrhea-what-to-do) - [Cat Diarrhea Treatment](/knowledge/veterinary-medicine/symptom-guides/cat-diarrhea-treatment) - [Cat Vomiting and Diarrhea: When to Worry](/knowledge/veterinary-medicine/digestive-and-urinary-care/cat-vomiting-diarrhea-when-to-worry) - [Cat Diarrhea: Causes, Home Care, and When to Worry](/knowledge/veterinary-medicine/online-vet-and-care-costs/cat-diarrhea-causes-home-care) - [Cat Diarrhea: When Should You Call the Vet?](/knowledge/veterinary-medicine/symptom-guides/cat-diarrhea-when-to-call-vet) - [Zyrtec for Cats Side Effects and Safety](/knowledge/veterinary-medicine/pet-medication-guide/zyrtec-cats-side-effects) - [Viral and Bacterial Diarrhea in Cats: Causes and Treatment](/knowledge/veterinary-medicine/digestive-and-urinary-care/cat-diarrhea-viral-bacterial) - [Can You Give a Cat Zyrtec? Dosage and Safety](/knowledge/veterinary-medicine/pet-medication-guide/zyrtec-for-cats-dosage-safety) ## References 1. [Disorders of the Stomach and Intestines in Cats - Cat Owners](https://www.merckvetmanual.com/cat-owners/digestive-disorders-of-cats/disorders-of-the-stomach-and-intestines-in-cats). Merck Veterinary Manual. 2. [Presence of Potential Enteropathogenic Bacteria in Cats and Association With Diarrhea in Multicat Households](https://pubmed.ncbi.nlm.nih.gov/40404601/). Journal of veterinary internal medicine, 2025. 3. [Gram-Negative Bacterial Overgrowth Predominated by: Potential Association with Intestinal Barrier Impairment and Inflammatory Diarrhea in Cats](https://pubmed.ncbi.nlm.nih.gov/42798011/). Veterinary sciences, 2026. 4. [Feline chronic diarrhea: causes and nutritional management](https://pubmed.ncbi.nlm.nih.gov/42502773/). Veterinary and animal science, 2026. 5. [Feline Coronavirus Prevalence in 493 Cats With Chronic Diarrhea](https://pubmed.ncbi.nlm.nih.gov/41771138/). Journal of veterinary internal medicine, 2025. 6. [Occurrence of Cystoisospora spp. and other intestinal parasites in dogs and cats with diarrhea](https://pubmed.ncbi.nlm.nih.gov/40690864/). Veterinary parasitology, 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: Cat Owners](https://www.merckvetmanual.com/cat-owners) - [Cornell Feline Health Center: Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information)