Cat medication side effects: what's normal and when to call the vet

Every medicine can cause side effects, and the FDA advises asking your vet what to watch for before you start. Most cat medication side effects are mild and short-lived. A few are emergencies. Knowing the difference means you won’t panic over a bit of drooling, and you won’t wait on a reaction that needs help now.

What are the most common cat medication side effects?

The most common problems involve the stomach and energy levels. The FDA lists vomiting, a smaller appetite, being less active and diarrhea as the side effects reported most often in cats and dogs on anti-inflammatory pain relievers, and many other drugs cause similar effects.

Common, usually mild effects include:

• Vomiting or soft stool, especially in the first few days
• Eating less than usual
• Sleepiness or wobbliness, with drugs that act on the brain, such as gabapentin
• Drooling or foaming right after a pill. Cornell’s pharmacy says this is normal and not harmful
• Drinking and peeing more, with steroids such as prednisolone

Mild isn’t the same as ignore. Note what you see and when, and tell your vet. Giving a dose with food, or changing the timing, can sometimes help, but ask first.

Which side effects need a vet now, and which can wait?

Use this table as a guide. If you’re unsure, call your vet or use our should I go to the vet tool.

“Monitor” means mention it at the next visit or by phone, unless it gets worse. If a mild effect lasts more than a couple of days, or your cat seems unwell in any other way, call sooner.

What does an allergic reaction to medicine look like in a cat?

An allergic reaction happens when the immune system overreacts to a drug. The Merck Veterinary Manual describes two main types.

Hives and swelling. Merck notes these usually appear within about 20 minutes of exposure to a drug, vaccine or other trigger. Hives are small raised bumps, often with the fur standing up over them, and they may itch. Swelling of the lips, nose or around the eyes is more serious, and can get bad enough that the cat can’t open its eyes. Hives and swelling are usually not life-threatening, but a vet should see your cat the same day.

Anaphylaxis. This is rare but life-threatening. Signs usually start within seconds to minutes and can include:

• Severe difficulty breathing
• Sudden vomiting or diarrhea
• Heavy drooling, excitement or wobbliness
• Very pale gums and cold legs
• Collapse, seizures or coma

Merck calls anaphylaxis an extreme emergency. Go to the nearest vet immediately. See our emergency guide for what to do on the way. Tell your vet about any past reaction, so it’s on your cat’s record.

Which medicines need blood tests to check for side effects?

Some side effects don’t show on the outside. The FDA notes that some medicines can affect the kidneys or liver, so vets recommend regular blood tests for cats on long-term treatment. Two common examples:

Anti-inflammatory pain relievers (NSAIDs). These can cause stomach ulcers, kidney damage and liver damage. The FDA says cats are especially sensitive to NSAID side effects, and that most NSAID-related liver damage appears within the first three weeks. The AAFP advises that cats on NSAIDs must eat and drink well. If your cat isn’t eating or drinking normally, call your vet before giving the next dose. Your vet may recommend bloodwork and more frequent visits during treatment. See our guide to meloxicam for cats.

Methimazole for hyperthyroidism. VCA lists vomiting, poor appetite and tiredness as common side effects, usually in the first three months. Rarer problems include itching and facial scratches, serious drops in blood cell counts and liver disease. Merck notes that because most side effects appear in the first three months, vets check blood counts and thyroid levels often during that time. Cornell adds that regular blood tests also track kidney function. See our guides to methimazole and hyperthyroidism in cats.

Keep every recheck appointment, even if your cat seems fine. A blood test can catch a problem before your cat feels ill.

Can supplements and other medicines cause interactions?

Yes. Drug interactions happen when one product changes how another works, or when two add up to more side effects.

• Two NSAIDs, or an NSAID with a steroid. The FDA warns this combination raises the risk of stomach and gut damage and should be avoided.
• Methimazole can interact with several drugs, including some dewormers, heart medicines and phenobarbital, according to VCA.
• Gabapentin with opioid pain relievers, such as morphine. VCA advises using these together with caution.
• Supplements count. The AAFP notes that combining NSAIDs with some other medicines or supplements can cause side effects.

Tell your vet about everything your cat gets: prescriptions from other clinics, flea and worm products, vitamins, herbal remedies, calming products, CBD and special diets. Never add a human medicine. Some, like acetaminophen (paracetamol), can be fatal to cats. See our guide to human medications toxic to cats.

Can the way a pill is given cause side effects?

Sometimes. Tablets and capsules given without water can stick in a cat’s esophagus and burn its lining. In one report, five cats given clindamycin capsules without food or water developed trouble swallowing, regurgitation or gagging within days, and some went on to develop a narrowed esophagus. Following each pill with a little water or a treat helps. Our guide on how to give a cat a pill shows how.

How do I report a side effect?

Start with your vet. They can judge whether the problem is linked to the medicine, change the treatment and record it. They can often file a report for you.

Report it to the FDA. The FDA’s Center for Veterinary Medicine collects reports of side effects, medication errors, products that don’t work and product defects. It uses them to monitor drug safety after approval.

• For an FDA-approved animal drug, the FDA recommends calling the drug company on the number on the label and asking for technical services. The company must pass the report to the FDA. Approved drugs carry an “Approved by FDA under NADA” or “ANADA” number on the label.
• For other products, including human drugs used in cats and compounded medicines, fill in Form FDA 1932a from the FDA’s reporting page and email it to the address given there. You can also use this form for approved drugs.
• Include details: your cat’s health before the reaction, every drug and supplement with its dose, test results, and food and treats.
• Vaccines and some flea products are handled by other agencies. The FDA’s page explains where to send those.

Questions about reporting can go to the FDA at 1-888-FDA-VETS (1-888-332-8387). Tracking side effects and doses is easier with a medication log, which our cat medication dosage guide explains. For more drug guides, browse our cat medications guides.

Written by the CatsMed Team from the sources above. Reviewed by Dr. Jarod Grantland, DVM. It isn’t a substitute for seeing a vet. See how we research.