Onsior is not an antibiotic. It is a nonsteroidal anti-inflammatory drug (NSAID) used in veterinary medicine to control pain and inflammation in cats and dogs. Its active ingredient, robenacoxib, works by reducing inflammation and pain signaling rather than fighting bacterial infections.
What Onsior Actually Does
Antibiotics kill or stop the growth of bacteria. Onsior does something completely different. It blocks a specific enzyme in the body responsible for producing chemicals that cause pain, swelling, and fever. More specifically, robenacoxib is a highly selective COX-2 inhibitor, meaning it targets the enzyme most directly involved in inflammation while largely leaving alone the related COX-1 enzyme that helps protect the stomach lining and supports kidney function.
In lab testing, robenacoxib showed a selectivity ratio of over 967 to 1 for its target enzyme versus the protective one. That selectivity is the reason veterinarians consider it a relatively targeted pain reliever compared to older NSAIDs. It binds tightly to the inflammation-causing enzyme for about 25 minutes, while its attachment to the protective enzyme releases almost immediately. The practical result: it reduces swelling, lowers fever, and blocks pain at the site of injury or surgery.
What Onsior Is Prescribed For
The FDA has approved Onsior tablets for cats to control postoperative pain and inflammation after orthopedic surgery, spays, and neuters. Cats must weigh at least 5.5 pounds and be at least 4 months old. The standard dose is 1 mg/kg given once daily by mouth, and it is limited to a maximum of three days of use. An injectable form is also available, and vets can alternate between the injection and tablet as long as total treatment stays within three doses over three days.
Onsior is also used in dogs, though the approved indications and duration differ slightly between species. In both cases, the goal is short-term pain control, not infection treatment. If your pet has a bacterial infection after surgery, your vet would prescribe a separate antibiotic alongside Onsior, not instead of it.
Why the Confusion Happens
It’s common for pet owners to receive multiple medications after a surgery and lose track of which drug does what. Onsior tablets look like any other small pill, so there’s no obvious visual cue that it’s a pain reliever rather than an antibiotic. Vets also sometimes prescribe both an antibiotic and Onsior at the same time, which can blur the line between the two in a pet owner’s mind. The key distinction: Onsior handles pain and swelling, while antibiotics handle infection.
Important Safety Considerations
Because Onsior is an NSAID, it carries the same general category of risks as drugs like ibuprofen or aspirin in humans. It should never be combined with other NSAIDs or with corticosteroids, as stacking anti-inflammatory drugs significantly raises the risk of stomach ulcers, kidney damage, or liver problems. Pets that are dehydrated, on diuretic therapy, or have existing kidney, heart, or liver issues face the highest risk of complications.
The three-day limit on Onsior for cats is strict and exists for safety reasons. Unlike some dog NSAIDs that can be used for weeks or months, Onsior in cats is designed exclusively for short-term postoperative use. Giving it longer than directed or combining it with other anti-inflammatory medications can cause serious harm.
If your vet sent your pet home with Onsior and you’re unsure whether they also need an antibiotic, check the discharge paperwork or call the clinic. The two serve entirely different purposes, and one does not replace the other.