Onsior (robenacoxib) starts working in about 30 minutes in cats. That rapid onset is one of the reasons veterinarians reach for it when managing pain and inflammation after surgery or other procedures. A single tablet given once daily provides relief that lasts well beyond what you’d expect from a drug that clears the bloodstream quickly, thanks to the way it concentrates in inflamed tissue.
How Quickly You’ll See Results
After your cat swallows an Onsior tablet, the drug reaches peak levels in the blood within about 30 minutes. Pain relief, fever reduction, and anti-inflammatory effects all begin around that same half-hour mark. This is faster than many oral pain medications used in veterinary medicine, and it means your cat should start showing signs of comfort relatively soon after dosing.
What “comfort” looks like will depend on why your cat is taking the medication. After a spay or dental procedure, you may notice your cat becoming less restless, more willing to settle into a resting position, or less reactive when moving around. These changes can be subtle in cats, who tend to hide pain, so don’t expect dramatic behavioral shifts. A quiet, relaxed cat that’s willing to eat or sleep is generally a good sign.
Why It Lasts Longer Than Expected
Onsior has an unusual property that makes it especially effective for inflammation. The drug clears from the bloodstream fast, with a half-life of only about 1.7 hours after an oral dose. On paper, that sounds like it would wear off quickly. But robenacoxib preferentially migrates to inflamed tissue and stays there much longer. In inflammation models, the drug’s half-life at the site of inflammation was roughly 27 hours, compared to just 2.5 hours in blood.
This means the medication concentrates right where it’s needed and lingers there even after blood levels have dropped. It’s why a single daily dose provides meaningful pain control throughout the day, and it also helps explain the drug’s safety profile: lower circulating levels in the blood mean less exposure to organs like the kidneys and stomach.
How Onsior Reduces Pain and Swelling
Onsior belongs to the NSAID class of medications. It works by blocking the production of prostaglandins, chemicals your cat’s body releases at injury sites that cause pain, swelling, and fever. What sets it apart from older NSAIDs is its selectivity. It strongly targets the enzyme responsible for inflammation (COX-2) while largely sparing the enzyme that protects the stomach lining and supports kidney function (COX-1). At the standard dose for cats, it inhibits about 90% of inflammatory enzyme activity while affecting only about 5% of the protective enzyme over a 12-hour window.
Dosing and the 3-Day Limit
The standard oral dose for cats is one tablet once daily, dosed at about 1 mg per kilogram of body weight. Onsior tablets for cats come in a 6 mg size, and your vet will prescribe the appropriate amount based on your cat’s weight. The critical rule to know: Onsior is approved for a maximum of 3 consecutive days in cats. This isn’t a medication you continue indefinitely or refill on your own.
If your vet prescribed the injectable form (commonly given at the clinic right after surgery), that also counts toward the 3-day maximum. The injectable and tablet forms can be used interchangeably within that window. So if your cat received an injection at the clinic on surgery day, you may only have 1 or 2 days of tablets to give at home.
Onsior is approved for cats that are at least 4 months old and weigh at least 5.5 pounds. Kittens younger than this or underweight cats should not receive it.
Getting the Most From Each Dose
Because Onsior reaches peak blood levels so quickly on an empty stomach, the timing of food can matter. The labeled studies measured peak absorption at 30 minutes when the tablet was given without food. If your vet hasn’t given specific instructions about feeding, giving the tablet before or apart from a meal may allow the fastest onset. That said, follow whatever your vet recommended for your cat’s specific situation, especially if your cat has a sensitive stomach.
Consistency also helps. Giving each dose at roughly the same time each day keeps a steady rhythm of relief, particularly since the drug accumulates at inflamed sites. If you miss a dose or your cat spits out a tablet, don’t double up. Give the next dose at the regular time.
What to Watch For During Treatment
Most cats tolerate Onsior well over the short 3-day course, partly because of its selective design. Still, it’s an NSAID, and the same general precautions apply. Watch for decreased appetite, vomiting, diarrhea, or changes in litter box habits (either less urine or unusual color). Lethargy beyond what you’d expect from post-surgical recovery is also worth noting. These side effects are uncommon at the approved dose and duration, but they warrant a call to your vet if they appear.
Cats with pre-existing kidney disease, liver problems, or dehydration are at higher risk from any NSAID. Your vet will typically check bloodwork before prescribing Onsior to rule out these concerns, especially in older cats. If your cat is already taking another anti-inflammatory or a corticosteroid, Onsior should not be added on top of it.