By Dr. Michelle Frye, DVM · Published 2026-05-01
TL;DR. Onsior (robenacoxib) is a coxib-class NSAID FDA-approved for short-term use in cats and dogs. Cat label is up to 3 days for post-op pain. Dose 1 mg/kg once daily. Concentrates in inflamed tissue, clears the bloodstream quickly. Never with steroids or another NSAID.
The Hardest Patient to Manage Pain In
Cats have historically been hard to give pain medication to. They lack the liver enzymes to safely metabolize many of the NSAIDs we use freely in dogs. They are notoriously stoic, hiding pain until they are profoundly uncomfortable. Onsior was developed specifically for them — the first coxib-class NSAID with a feline label, designed to provide reliable analgesia with a margin of safety the species needs.
What Onsior Actually Does
Onsior (robenacoxib) is a coxib NSAID, structurally related to drugs like deracoxib but designed with two unusual features: it concentrates in inflamed tissues (where pain lives) and clears the bloodstream rapidly (where systemic side effects originate). The result is sustained tissue-level effect with minimal blood-level exposure.
- Cat dose: 1 mg/kg orally once daily, up to 3 consecutive days for post-op pain.
- Dog label: short-term and longer use for soft-tissue and orthopedic pain at veterinarian's direction.
- Forms: 6 mg flavored tablets (cat) and various tablet sizes (dog).
- Onset: 30–60 minutes.
Things People Are Wrong About
Myth 1: "It's safe long-term in cats." The U.S. cat label is up to 3 days. Off-label longer courses are used in some clinical situations, but only with veterinary monitoring.
Myth 2: "I can use it with prednisone." No. Like all NSAIDs, Onsior should never be combined with corticosteroids or other NSAIDs.
Myth 3: "Cats don't need pain medication after a spay." They do. Cats hide pain better than dogs but feel it just as much. Post-op analgesia improves recovery, eating, and demeanor.
Myth 4: "I can give the dog tablet, just smaller." The dog and cat formulations differ in tablet strength and intended dose. Use what your vet dispensed for the species.
When NOT to Use Onsior
Avoid in pets with significant kidney, liver, or bleeding disorders. Avoid in dehydrated patients (NSAIDs concentrate hazardously in low-volume states). Never combine with other NSAIDs or with corticosteroids. Use the species-specific formulation. Stop and call your veterinarian if your pet stops eating, vomits repeatedly, develops black or tarry stool, or seems unusually weak.
What I Tell Owners After 30 Years
For decades, post-spay cats went home on barely any pain control because we didn't have a safe NSAID for them. Onsior changed that. Used as labeled (short-term, single agent, well-hydrated patient), it provides excellent post-op analgesia with a remarkably good safety record. It is not a long-term arthritis drug for cats — that's a different conversation involving meloxicam dosing, gabapentin, and Solensia. But for the spay, the dental, the wound, the limp that resolves in three days — this is the right tool.
Frequently Asked Questions
Is Onsior safe for cats?
Yes, when used as labeled — the U.S. cat label is up to 3 days at 1 mg/kg once daily. Longer courses are sometimes used off-label under veterinary supervision.
How long does Onsior take to work?
Pain relief is typically visible within 30–60 minutes of dosing, with sustained tissue-level effect for the dosing interval.
Can dogs take Onsior?
Yes. Onsior is FDA-approved for soft-tissue surgery pain in dogs and is also used short-term for acute musculoskeletal disorders.
Can I give Onsior with prednisone?
No. Like all NSAIDs, Onsior should never be combined with corticosteroids. The combination dramatically raises GI ulceration and kidney risk.
What are the side effects of Onsior?
Most common are vomiting, soft stool, and decreased appetite. Rare serious effects include GI ulceration, kidney injury, and idiosyncratic liver reactions.
Can I use Onsior for my cat's arthritis long-term?
The U.S. label is short-term. Long-term feline arthritis pain is more commonly managed with low-dose meloxicam, gabapentin, Solensia (frunevetmab) injections, environmental modification, and weight management. Discuss with your veterinarian.
Is Onsior the same as Rimadyl?
No. Both are NSAIDs but different molecules. Robenacoxib (Onsior) is feline-labeled; carprofen (Rimadyl) is canine-only and unsafe in cats.
Where can I buy Onsior?
Onsior is prescription-only. Smarty Vets dispenses Onsior tablets from a licensed pharmacy.
This article is for educational purposes and is not a substitute for veterinary advice. Always consult your veterinarian before starting, stopping, or changing any medication for your pet.