Onsior (robenacoxib) occupies a specific, often misunderstood place in veterinary pain control. It is an FDA-approved NSAID, it is one of the very few NSAIDs approved for use in cats in the United States, and it is approved only for short-term, post-surgical use — a maximum of three days. Almost every mistake owners (and sometimes clinics) make with Onsior comes from ignoring one of those three facts: treating it like a daily arthritis drug, giving the dog tablet to a cat, or running it alongside another NSAID without a washout.

This article covers what the label actually says, how the dog and cat approvals differ, the side-effect and monitoring expectations, and where Onsior fits relative to carprofen, meloxicam, and grapiprant.

What Onsior is

Onsior is a nonsteroidal anti-inflammatory drug of the coxib class, meaning it preferentially blocks the COX-2 enzyme that drives inflammation and pain while relatively sparing the COX-1 enzyme that helps protect the stomach lining and maintain blood flow to organs. That COX-2 selectivity is the same principle behind celecoxib (Celebrex) in human medicine. Robenacoxib is the active ingredient; Onsior is the Elanco brand, and as of 2026 the first FDA-approved generic robenacoxib tablet (for cats, sponsored by ZyVet Animal Health) and a generic injection (Cronus Pharma) have also reached the market.

It comes as:

• Flavored oral tablets (different strengths and label instructions for dogs versus cats)
• An injectable solution (20 mg/mL) given subcutaneously by the veterinarian, typically at the time of surgery

It is available only by prescription from a licensed veterinarian.

The single most important thing: dog and cat labels are different

Onsior is approved for both species, but the labeled surgeries, the tablet strengths, and the warnings are not the same, and the dog and cat products are not interchangeable.

In dogs, Onsior tablets are approved for the control of postoperative pain and inflammation associated with soft tissue surgery (for example, spay, neuter, tumor removal, or abdominal surgery) in dogs weighing more than 5.5 lb (2.5 kg) and older than 4 months, for a maximum of three days. The dog label explicitly states "Do not use in cats" — cats cannot be accurately dosed with the tablet sizes made for dogs.

In cats, Onsior is approved for the control of postoperative pain and inflammation associated with orthopedic surgery, ovariohysterectomy (spay), and castration in cats at least 4 months old and weighing at least 5.5 lb, also for a maximum of three days. The injection can be given at the time of surgery, with oral tablets sent home for the next one to two days.

The injectable form is approved for both species and is typically given by the veterinarian around the time of surgery — in dogs, the first dose is given about 45 minutes before surgery alongside the pre-anesthetic medications, and subsequent doses can be given subcutaneously or swapped for the oral tablet, up to three total doses over three days.

This species split is not a technicality. Giving a dog-labeled tablet to a cat, or using the cat-labeled schedule for a dog's soft-tissue procedure, is outside the label and can result in inaccurate dosing.

Why the three-day limit exists — and why cats are a special case

Onsior is approved for a maximum of three days in both species in the United States. "Safety not demonstrated for longer than 3 days" is written directly into the label, and there is a reason: in a month-long pilot study, a small number of dogs developed serious liver toxicity, and foreign post-marketing experience linked long-term robenacoxib use in dogs to elevated liver enzymes, hepatic necrosis, and death. The short, three-day course is where the safety data are strongest.

For cats, the three-day ceiling reflects a broader, harder constraint. The FDA is explicit: only two NSAIDs are approved for short-term use in cats in the U.S. — meloxicam (as a single pre-surgical injection) and robenacoxib (for up to three days as a tablet or injection) — and no NSAID is approved for long-term use in cats. More than a single dose of meloxicam in cats (an extra-label practice) has caused kidney failure and death, and more than three doses of robenacoxib in cats have not been shown to be safe. This is why a cat sent home with Onsior gets two or three tablets, not a refillable bottle.

(Outside the U.S., robenacoxib carries an additional label for chronic osteoarthritis in dogs. U.S. veterinarians sometimes use it off-label for musculoskeletal pain in both species, but that is an extra-label decision made patient by patient — not the approved use, and not appropriate for owners to pursue on their own.)

How dosing works

The approved dose of Onsior injection and tablets in both dogs and cats is approximately 0.91 mg per pound (about 2 mg/kg) once daily, for a maximum of three days, not exceeding one dose per day. Tablets are flavored and taken voluntarily by most animals, and may be given with or without food. The specific tablet count, strength, and schedule come from the veterinarian's prescription; the dose is set by species, body weight, and the procedure, and owners should not adjust it.

Side effects

In the U.S. field studies, the most commonly reported adverse reactions in dogs were diarrhea, vomiting, and decreased appetite. Less common but serious events associated with Onsior have included hepatopathy (liver injury), ataxia, skin reactions/urticaria, and anaphylaxis. In anesthetized dogs, bradycardia, second-degree heart block, and ventricular arrhythmia have been reported. In cats, the most commonly reported adverse reactions were increased bleeding at the incision site, elevated white blood cell count (leukocytosis), fever, inappetence, infected incision sites, vomiting, and lethargy.

As with any NSAID, serious adverse reactions can occur without warning and, in rare cases, lead to death. Stop the drug and contact the veterinarian immediately if you see:

• Decreased appetite, or vomiting or diarrhea (especially black, tarry, or bloody stools)
• Yellowing of the gums, skin, or whites of the eyes (jaundice — a liver sign)
• Changes in drinking or urination (frequency, amount, or accidents — a kidney sign)
• Incoordination, weakness, stumbling, or seizures
• Marked change in behavior or activity level
• Skin swelling, hives, or facial swelling

NSAID effects can persist longer than expected in animals with liver or kidney disease.

Contraindications, cautions, and the NSAID washout

Onsior should not be used in animals who:

• Are allergic to robenacoxib or have a known intolerance to other NSAIDs or aspirin
• Have gastrointestinal ulcers, or bloody vomit or stool
• Are presently taking another NSAID or a corticosteroid (such as prednisolone) — see the washout note below
• Have significant kidney, liver, or heart disease, or a bleeding disorder such as von Willebrand disease
• Are dehydrated, hypovolemic, or on diuretic therapy
• Are pregnant, nursing, or intended for breeding
• Weigh less than 5.5 lb or are younger than 4 months

Geriatric animals, and those with conditions that predispose to dehydration, warrant extra caution.

The NSAID washout is the rule owners most often break. Onsior must not be given at the same time as another NSAID or a corticosteroid, and the label calls for an appropriate washout period when switching from one NSAID to another or from a steroid to an NSAID. Practically, this means: tell the veterinarian about every pain or anti-inflammatory medication given in the prior days to weeks, including over-the-counter products, leftover carprofen or meloxicam, aspirin, or a steroid prescribed for a different problem. Human NSAIDs (ibuprofen, naproxen, aspirin) are a separate toxicity concern in pets and should never be given without veterinary direction. Drug compatibility with common protein-bound medications (certain cardiac, anticonvulsant, and behavioral drugs) has not been fully studied, so a complete medication list matters here too.

Monitoring

The FDA and the label recommend that animals undergo a thorough history and physical exam, with baseline blood work (kidney and liver values, and a blood count) before NSAID therapy is started, particularly in middle-aged and senior patients. For a three-day post-surgical course in a young, healthy animal, the pre-anesthetic bloodwork often fills this role; for older animals or any longer off-label course, periodic rechecks are appropriate. Owners should observe closely for the side-effect signs listed above, especially decreased appetite — often the first hint of NSAID intolerance.

How Onsior compares with other NSAIDs

Onsior is a perioperative, short-course NSAID. The most common point of confusion is treating it like the daily arthritis drugs.

• Carprofen (Rimadyl, Novox, Vetprofen) — the workhorse daily NSAID for canine osteoarthritis and post-op pain; long-term labeled use in dogs. It is dogs-only at labeled doses.
• Meloxicam (Metacam) — daily OA NSAID in dogs; in the U.S., cats get only a single pre-surgical injection. Where Onsior is the multi-day cat option (up to three days), meloxicam is the single-dose cat option.
• Grapiprant (Galliprant) — a newer daily canine OA drug that works through the EP4 prostaglandin receptor rather than COX inhibition; a daily OA option that some clinicians choose to spare COX/renal effects, though it is dogs-only.
• Firocoxib (Previcox), deracoxib (Deramaxx) — additional daily canine OA NSAIDs.

Onsior's niche is the short post-surgical window in both species, including cats — a space the daily OA NSAIDs are not built for. In a randomized non-inferiority trial in dogs undergoing soft-tissue surgery, a single pre-surgical Onsior injection followed by oral tablets provided pain control equivalent to meloxicam, with comparable tolerability — which is the evidence behind using it as a short post-op course. If the need is long-term canine arthritis pain, a different drug is usually a better fit; if the need is a few days of post-op comfort in a cat, Onsior (or a single meloxicam injection) is one of the very few approved options.

What to ask your veterinarian

• Which species-specific product and schedule is being sent home, and exactly how many doses? (It should be a short course.)
• What bloodwork was done before surgery, and does my pet need a recheck? Kidney and liver baselines matter, especially in older animals.
• What other medications has my pet had recently — including any NSAID, steroid, aspirin, or supplement — so the washout is handled correctly?
• What signs should make me stop the drug and call? Decreased appetite, vomiting, diarrhea, jaundice, or changes in drinking/urination are the priority signals.
• For ongoing arthritis pain in my dog or cat, what is the long-term plan? Onsior is a bridge around surgery, not a chronic OA regimen; a separate strategy is needed for chronic pain.

Used within its label — the right species product, a three-day course, a clean medication history, and a watch for decreased appetite — Onsior is a safe and useful perioperative NSAID and one of the few pain options a cat can take home from surgery. The danger is in treating it as interchangeable across species or as a long-term arthritis drug, which the label was specifically written to prevent.

Sources

• FDA — What Veterinarians Should Advise Clients About Pain Control and NSAIDs in Dogs and Cats (the two cat-approved NSAIDs; no long-term cat NSAIDs; 3-day robenacoxib limit): https://www.fda.gov/animal-veterinary/resources-you/what-veterinarians-should-advise-clients-about-pain-control-and-nonsteroidal-anti-inflammatory-drugs
• FDA — FDA Approves First Generic Robenacoxib Tablet for Postoperative Pain & Inflammation in Cats (Jan 9, 2026): https://www.fda.gov/animal-veterinary/cvm-updates/fda-approves-first-generic-robenacoxib-tablet-postoperative-pain-inflammation-cats
• DailyMed — Onsior (robenacoxib) injection label (cat and dog indications, dose, contraindications, adverse reactions): https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=f4f307a0-88b5-4740-b198-35edef954bf1
• FDA/Elanco Onsior (robenacoxib) tablet Client Information Sheet for dogs (soft-tissue surgery label; "Do not use in cats"; adverse reactions): https://downloads.regulations.gov/FDA-2021-P-1085-0003/content.pdf
• dvm360 — First generic robenacoxib injectable is FDA-approved for postoperative use in dogs and cats (approval history: dogs 2016, cats 2015; 0.91 mg/lb dosing): https://www.dvm360.com/view/first-generic-robenacoxib-injectable-is-fda-approved-for-postoperative-use-in-dogs-and-cats
• VCA Animal Hospitals — Robenacoxib (indications, side effects, contraindications, monitoring): https://vcahospitals.com/know-your-pet/robenacoxib
• Drugs.com — Onsior Tablets for dogs (adverse reactions, hepatopathy in 28-day study, contraindications): https://www.drugs.com/vet/onsior-tablets-for-dogs.html
• European Medicines Agency — Onsior (robenacoxib) summary (EU chronic canine OA label; overdose and contraindication data): https://ec.europa.eu/health/documents/community-register/2015/20150918132991/anx_132991_en.pdf
• PMC — Robenacoxib versus meloxicam for peri-operative pain in soft-tissue surgery in dogs (randomized non-inferiority trial): https://pmc.ncbi.nlm.nih.gov/articles/PMC3655053/