A single Onsior injection provides approximately 24 hours of pain relief in cats, which is why it’s dosed once daily. The drug works fast, reaching peak levels in the blood within about one hour, and its effects at the site of inflammation last considerably longer than its presence in the bloodstream suggests.
Why It Lasts a Full Day
Onsior (robenacoxib) clears from a cat’s blood relatively quickly, with a half-life of only about 2.5 hours. That might sound like the pain relief would wear off fast, but the drug behaves differently in inflamed tissue. It concentrates at the site of pain and inflammation, where its half-life jumps to around 27 hours. This is what allows a single injection to cover a full 24-hour period despite disappearing from the bloodstream much sooner.
Pain relief begins roughly 30 minutes after the injection, making it a common choice for controlling pain before, during, and after surgery. Peak blood concentrations hit at about one hour.
How Many Injections Can a Cat Receive?
The FDA-approved protocol allows one injection per day for a maximum of three consecutive days. Each injection is given under the skin at a dose of 2 mg/kg. This short treatment window is typical for the injectable form, which is designed primarily for post-surgical pain from procedures like spaying, neutering, or orthopedic surgery.
If your cat needs pain management beyond three days, the injectable and tablet forms of Onsior can be used interchangeably within that three-day window, though the doses are slightly different between the two forms. Your vet will manage that transition. Cats must be at least 4 months old and weigh at least 5.5 pounds to receive either form.
What to Watch for After the Injection
Onsior is a nonsteroidal anti-inflammatory drug (NSAID), and like all NSAIDs, it can affect the kidneys, liver, and stomach. The most commonly reported side effects in cats are loss of appetite, lethargy, and vomiting. Less common but more serious reactions include elevated kidney values, kidney insufficiency, diarrhea, weight loss, and dehydration. In rare post-approval reports, some cats developed kidney failure, and some of those cases were fatal.
Cats that are dehydrated, have pre-existing kidney or liver problems, or are on other medications that stress the kidneys are at the highest risk for complications. This is one reason vets typically run bloodwork before surgery and use IV fluids during the procedure when Onsior is part of the pain plan. If your cat seems unusually sluggish, stops eating, or vomits after receiving the injection, contact your vet rather than waiting for the next scheduled dose.
How Onsior Compares to Oral Tablets
The injectable form is almost exclusively a veterinary clinic medication. You won’t be giving injections at home. If your cat needs continued pain relief after coming home from surgery, your vet may send you home with Onsior tablets instead. The tablets are dosed slightly higher on a per-kilogram basis (about 2.4 mg/kg orally versus 2 mg/kg for the injection), but they follow the same once-daily, maximum-three-day rule in cats.
Both forms target the same inflammation pathways and provide similar duration of relief. The injection simply offers faster onset, which is why it’s preferred around the time of surgery. By the time your cat is home and eating normally, tablets can take over if additional days of pain control are needed.