A single dose of Onsior (robenacoxib) clears from a cat’s bloodstream quickly, with a half-life of only about 1.1 hours. But the pain-relieving effect lasts much longer than that blood level suggests, which is why it’s dosed just once daily. The drug concentrates in inflamed tissues, where it persists with a half-life of roughly 27 hours, providing a full day of pain and inflammation control from each dose.
Why It Works Longer Than It Stays in the Blood
Onsior behaves differently from what you might expect based on standard drug timelines. After a tablet or injection, robenacoxib binds tightly to proteins in the blood (over 98% of the drug attaches to plasma proteins). When tissue is inflamed, protein-rich fluid leaks into that area from the bloodstream. Because robenacoxib hitches a ride on those proteins, it gets carried directly to the site of inflammation and accumulates there in high concentrations.
The result is a drug that vanishes from the general circulation within a few hours but stays active right where the pain is for much longer. In studies measuring drug levels in inflamed tissue versus blood, the half-life at the inflammation site was about 27 hours compared to just 2.5 hours in the blood. This tissue-targeting effect is what makes once-daily dosing effective and also helps explain why Onsior causes fewer body-wide side effects than older anti-inflammatory drugs: it spends most of its time concentrated at the problem area rather than circulating throughout the body.
How It’s Dosed
Onsior tablets for cats are given once daily at a dose of roughly 1 mg per kilogram of body weight (0.45 mg per pound). The tablets come in a 6 mg size, and dosing is straightforward:
- Cats 5.5 to 13.2 lbs: 1 tablet once daily
- Cats 13.3 to 26.4 lbs: 2 tablets once daily
Cats under 5.5 pounds or younger than 4 months old should not receive Onsior, because the tablet can’t be split accurately enough for their size. The injectable form, given under the skin by a veterinarian, has a similar half-life of about 1.1 hours in the blood but the same extended tissue persistence.
The 3-Day Maximum
Onsior is FDA-approved in cats for a maximum of 3 consecutive days. It’s specifically indicated for controlling pain and inflammation after orthopedic surgery, spaying, or neutering. Your cat can receive up to 3 total doses over those 3 days, no more than one dose per day. The doses can be a mix of tablets and injections if your vet started with a shot and then sent you home with pills.
This 3-day limit is a key difference between cats and dogs. Dogs can take Onsior for longer periods, but cats metabolize many anti-inflammatory drugs differently, and the short approved window reflects the safety data available. Some veterinarians may prescribe it off-label for longer use in certain situations, but the FDA label is firm on the 3-day cap for feline patients.
What This Means After Your Cat’s Last Dose
After your cat takes the final dose, the drug will clear from the bloodstream within a few hours. However, the anti-inflammatory effect at the surgical or injury site will taper off more gradually over the following day, thanks to that 27-hour tissue half-life. In practical terms, you can expect meaningful pain control to continue for roughly 24 hours after the last tablet or injection, with the effect slowly diminishing after that.
If your cat seems uncomfortable or is showing signs of pain once Onsior has worn off, that’s worth a call to your vet. After a routine spay or neuter, 3 days of pain management is typically sufficient for most cats to get through the worst of the postoperative discomfort. For orthopedic procedures, your vet may want to transition to a different pain management approach once the Onsior course is complete.
Side Effects to Watch For
Because robenacoxib selectively targets the enzyme involved in inflammation (rather than broadly affecting related enzymes that protect the stomach lining and kidneys), it tends to be well tolerated over a short course. The most commonly reported side effects in cats include soft stool, vomiting, and decreased appetite. These are generally mild and resolve once the medication is stopped. If your cat stops eating entirely, vomits repeatedly, or seems more lethargic rather than less after taking Onsior, contact your vet rather than giving the next dose.