Onsior is FDA-approved and generally safe for cats when used as directed, but with a strict limit: no more than three consecutive days. It’s a prescription anti-inflammatory pain reliever in the coxib class, specifically approved for managing pain and inflammation after spay, neuter, and declaw surgeries in cats weighing at least 5.5 pounds and at least 4 months old.
What Onsior Does
Onsior (robenacoxib) is a non-steroidal anti-inflammatory drug, or NSAID, designed to target the enzymes responsible for pain and swelling at surgical sites. Unlike some older NSAIDs that affect the whole body broadly, robenacoxib belongs to the coxib class, which is more selective in how it blocks inflammation. This selectivity is part of why it was approved for cats, a species notoriously sensitive to many common pain medications.
The approved dose is 1 mg per kilogram of body weight, given by mouth once daily. It comes as a 6 mg tablet, and your vet will determine the correct dose based on your cat’s weight. Onsior can also be given as an injection at the veterinary clinic, and the two forms can be used interchangeably within that three-day window.
Side Effects From Clinical Trials
In the FDA field study used for approval, 167 cats received Onsior after surgery while 82 received a placebo. The side effects were relatively mild and occurred at low rates:
- Loss of appetite or weight loss: 4 out of 167 cats on Onsior (compared to 2 on placebo)
- Vomiting: 4 out of 167 cats (compared to 1 on placebo)
- Diarrhea or bloody stool: 3 out of 167 cats (compared to 1 on placebo)
These numbers mean roughly 2% to 2.4% of cats experienced any given side effect. The fact that some cats on placebo also showed these signs suggests that surgery itself, anesthesia, and the stress of a clinic visit contribute to digestive upset. Still, if your cat stops eating, vomits repeatedly, or has bloody stool after taking Onsior, contact your vet rather than giving the next dose.
The Three-Day Limit Matters
The most important safety rule with Onsior in cats is the maximum three-day duration. This isn’t a loose guideline. The FDA approval is specifically limited to three days because cats metabolize NSAIDs differently than dogs or humans, and longer use raises the risk of kidney or gastrointestinal damage. If your cat still seems painful after three days, your vet will need to explore other pain management options rather than continuing Onsior.
This is a meaningful difference from how the drug is used in dogs, where longer courses are sometimes prescribed. For cats, the short window is a core part of what makes the drug safe.
Cats That Should Not Take Onsior
Onsior is not appropriate for every cat. Your vet should avoid prescribing it if your cat is under 4 months old, weighs less than 5.5 pounds, or has pre-existing kidney disease. Because NSAIDs reduce blood flow to the kidneys, cats with any degree of kidney impairment are at higher risk for serious complications. Cats that are dehydrated, have low blood pressure, or have liver problems also face elevated risk.
If your cat has a heart condition, this is worth flagging to your vet as well. Robenacoxib can affect the heart’s electrical rhythm (specifically the QT interval), so combining it with certain medications that have similar effects could be problematic.
Medications That Don’t Mix With Onsior
Several common drug categories must not be used at the same time as Onsior:
- Other NSAIDs: Combining two anti-inflammatory drugs dramatically increases the risk of stomach ulcers and kidney injury. If your cat was recently on a different NSAID, your vet needs to allow a washout period before starting Onsior.
- Corticosteroids: Steroids like prednisolone, sometimes prescribed for allergies or inflammatory conditions, should not overlap with Onsior. The same washout principle applies when switching between them.
- Kidney-stressing drugs: Any medication that puts additional strain on the kidneys requires careful monitoring if used alongside Onsior.
- Certain antihistamines, gut motility drugs, and anesthetics: These can affect heart rhythm in ways that compound Onsior’s own effects on the heart’s electrical activity.
If your cat takes any regular medications, make sure your vet has the full list before Onsior is prescribed.
What to Watch for at Home
Most cats tolerate Onsior well for the short period they’re on it, especially since they’re typically recovering from surgery and already under close observation. The signs to watch for mirror common NSAID side effects: vomiting, refusal to eat, dark or tarry stool, increased thirst or urination, and unusual lethargy beyond what you’d expect from post-surgical recovery.
Keep in mind that some post-surgical sluggishness and reduced appetite is normal in the first 24 hours. What you’re looking for is a cat that seems to be getting worse rather than gradually improving, or any symptom that appears suddenly after the first dose or two. If your cat vomits within an hour of taking a tablet, let your vet know before giving the next one, as they may adjust the plan or switch to the injectable form.