Onsior is a prescription anti-inflammatory painkiller for cats, used to control pain and swelling after surgery. Its active ingredient, robenacoxib, belongs to a class of drugs called coxibs, which target inflammation more selectively than older painkillers. In the United States, Onsior is FDA-approved for short-term use only, with a strict maximum of 3 consecutive days.
What Onsior Is Approved to Treat
In the U.S., Onsior is specifically approved for postoperative pain and inflammation in cats following orthopedic surgery, spays, and neuters. The FDA label limits its use to cats weighing at least 5.5 pounds and at least 4 months old. Clinical trials showed it was effective and well-tolerated for managing pain after these common procedures when given for no more than three days.
In Europe, the approved uses are somewhat broader. The European Medicines Agency has authorized Onsior for acute pain and inflammation related to musculoskeletal disorders in cats, not just post-surgical pain. This is why some veterinarians outside the U.S. may prescribe it for different reasons than what you’d see domestically.
How It Works
Robenacoxib is a selective COX-2 inhibitor. Your cat’s body produces two versions of an enzyme called cyclooxygenase: COX-1 and COX-2. COX-1 handles everyday housekeeping jobs like protecting the stomach lining and maintaining blood flow to the kidneys. COX-2 ramps up at injury sites and drives inflammation, swelling, and pain.
Older anti-inflammatory drugs block both enzymes, which is why they can cause stomach upset and kidney problems. Robenacoxib preferentially blocks COX-2 while largely leaving COX-1 alone. This targeted approach is designed to reduce pain and inflammation with a lower risk of gastrointestinal and kidney side effects, at least over short treatment periods.
Tablet and Injection Forms
Onsior comes in two forms for cats: a 6 mg oral tablet and a subcutaneous injection. The injection is given by a veterinarian, typically as the first dose right around surgery. After that, your vet may send you home with tablets to continue treatment, or they may give additional injections at the clinic. The two forms can be used interchangeably, as long as the total course doesn’t exceed 3 doses over 3 days, with no more than one dose per day.
The tablet dose is 1 mg per kilogram of body weight, given once daily by mouth. The injection dose is slightly higher at 2 mg per kilogram. Both are prescription-only.
Side Effects to Watch For
In FDA clinical trials for the short-term (3-day) indication, Onsior was generally well tolerated. However, European data from longer studies paints a fuller picture of what the drug can do. In field studies where cats received Onsior for up to 12 weeks for chronic musculoskeletal problems, vomiting was the most frequently reported side effect. Loss of appetite, diarrhea, lethargy, and defecating outside the litter box were also commonly observed.
For the typical 3-day post-surgical course, serious side effects are uncommon. But if your cat stops eating, seems unusually tired, or vomits repeatedly during treatment, those are signs worth flagging for your vet right away.
Cats Who Should Not Take Onsior
Onsior carries extra risk for cats with existing heart, kidney, or liver problems. Cats who are dehydrated or have low blood pressure are also poor candidates, because anti-inflammatory drugs can reduce blood flow to the kidneys and worsen these conditions. If your cat has any of these issues, your vet will weigh the risks carefully before prescribing it.
Like all NSAIDs, Onsior should not be combined with other anti-inflammatory drugs or corticosteroids. Stacking these medications dramatically increases the chance of gastrointestinal ulceration and kidney damage. If your cat is already taking any pain or anti-inflammatory medication, make sure your vet knows before starting Onsior. An overdose of any NSAID can cause gastrointestinal, kidney, or liver toxicity, particularly in cats that are already compromised.
Why It’s Limited to 3 Days in the U.S.
The 3-day cap isn’t arbitrary. A 6-month safety study tested robenacoxib in young cats at the recommended dose and at 3 and 5 times the recommended dose. Researchers observed reduced body weight and food intake starting around 3 weeks in, along with changes in heart rhythm, reduced kidney weight, mild kidney lesions (including tissue damage and chronic inflammation), and mild liver changes. These findings appeared dose-related and possibly time-related.
Regulators concluded that a connection between robenacoxib and these effects couldn’t be ruled out. Because the U.S. approval only covers short-term use, the FDA determined these long-term findings didn’t directly apply to the 3-day label. But they help explain why the drug isn’t approved for extended courses in cats in the United States, and why exceeding the prescribed duration is a genuine safety concern rather than just legal caution.
What to Expect When Your Cat Takes Onsior
If your cat is coming home from a spay, neuter, or orthopedic procedure with Onsior tablets, you’ll typically give one tablet per day for up to 3 days. The tablet is small and flavored, though some cats still resist taking it. Give it at roughly the same time each day. You don’t need to give it with food, but offering it alongside a small meal is fine if that makes dosing easier.
During the treatment period, keep an eye on your cat’s appetite, energy level, and litter box habits. Most cats tolerate the short course without any noticeable problems and simply recover from surgery with less visible discomfort. If your cat seems to still be in pain after the 3-day course ends, contact your vet rather than continuing the medication on your own. There are other pain management options your vet can consider for longer recovery periods.