Previcox is a brand-name anti-inflammatory medication containing the active ingredient firocoxib. It’s FDA-approved for dogs, not horses, but many horse owners encounter the name because veterinarians sometimes prescribe it off-label for equine pain and inflammation, particularly from osteoarthritis. The horse-approved version of firocoxib is sold under the brand name Equioxx, which contains the same active ingredient at the same concentration but is specifically tested and labeled for equine use.
How Firocoxib Works
Firocoxib belongs to a class of drugs called coxibs. It’s a non-steroidal anti-inflammatory drug (NSAID) that works by selectively blocking an enzyme called COX-2, which your horse’s body ramps up during inflammation. This reduces pain, swelling, and fever. What makes firocoxib different from older equine NSAIDs like phenylbutazone (“bute”) is its selectivity. It largely spares COX-1, an enzyme involved in protecting the stomach lining and maintaining normal kidney function. In theory, this selectivity means fewer gastrointestinal side effects, though the FDA notes the real-world significance of this selectivity hasn’t been fully established.
What It Treats in Horses
Firocoxib is primarily prescribed to manage pain and inflammation associated with osteoarthritis in horses. In a clinical trial of 253 horses with naturally occurring osteoarthritis, about 85% of horses treated with firocoxib showed clinical improvement after 14 days of treatment. That success rate was comparable to phenylbutazone, which has been the go-to equine painkiller for decades. Firocoxib actually outperformed bute on several specific measures: horses showed greater improvement in pain on joint manipulation, joint circumference, and range of motion.
Veterinarians also use firocoxib for post-surgical pain, soft tissue injuries, and other inflammatory conditions, though its core labeled use is for osteoarthritis.
Previcox vs. Equioxx
This is where things get important for horse owners. Previcox is the canine formulation of firocoxib. Equioxx is the equine formulation. Both contain firocoxib, but Previcox is significantly cheaper, which is why many horse owners ask about using it. However, using the dog product in horses is technically illegal under FDA regulations. You can legally use an approved human drug in an animal even if a veterinary version exists, but you cannot substitute an animal drug approved for one species in another species simply because it costs less.
That said, veterinarians do have some legal latitude to prescribe drugs off-label under specific regulatory guidelines. If your vet has dispensed Previcox for your horse, they’ve made a clinical judgment within their professional authority, but it’s worth understanding that Equioxx (and now generic firocoxib tablets approved for horses) went through equine-specific safety testing that Previcox did not.
Dosage for Horses
The recommended dose of firocoxib for horses is one 57 mg tablet once daily for horses weighing 800 to 1,300 pounds. The standard treatment duration is up to 14 days. This works out to roughly 0.1 mg per kilogram of body weight, which is the dose used in clinical trials. Previcox tablets come in 57 mg and 227 mg sizes (the latter designed for large dogs), which is part of why horse owners gravitate toward them, but the 227 mg tablet would deliver a significant overdose to most horses if given whole.
Side Effects and Risks
Like all NSAIDs, firocoxib can cause gastrointestinal, kidney, and liver problems. The most characteristic side effects in horses involve the mouth: ulcers and erosions on the tongue, lips, and gums. In safety studies, even horses receiving the standard dose showed some increase in oral ulcers compared to untreated horses, and the drug delayed healing of pre-existing mouth sores across all dose levels tested.
Kidney damage is the more serious concern. In toxicity studies, horses developed a condition called tubulointerstitial nephropathy (progressive kidney inflammation) at doses as low as the recommended level, though clinically significant kidney damage was more common at higher doses. At three to five times the normal dose, some horses showed elevated markers of kidney stress and structural kidney damage including tissue death in the kidney’s filtering structures.
Stomach ulcers, a well-known risk with equine NSAIDs generally, also appeared at elevated doses. You should never combine firocoxib with other NSAIDs or corticosteroids, as this dramatically increases the risk of gastrointestinal ulceration and perforation.
Signs of Overdose
If a horse receives too much firocoxib, the earliest visible signs are typically oral ulcers on the lips, tongue, and gums, along with skin erosions around the jaw and head. Prolonged overdose can cause elevated liver enzymes, kidney hemorrhage, gastric ulcers, and prolonged bleeding times. There is no antidote for firocoxib overdose. Treatment is supportive: intravenous fluids, stomach protectants, and monitoring bloodwork.
Long-Term Use
The labeled recommendation caps treatment at 14 consecutive days, but many horses with chronic osteoarthritis need longer pain management. A preliminary study followed seven healthy horses given daily firocoxib for 40 days. None developed clinical signs of gastrointestinal toxicity like abdominal pain, reduced appetite, oral ulcers, or diarrhea. Some blood values shifted by the end of treatment, including slight decreases in red blood cell counts, liver enzymes, and creatinine, but all stayed within normal reference ranges and returned to baseline within 30 days of stopping the drug.
These results are encouraging but come from a small study of healthy horses. Horses with pre-existing kidney or liver problems, or those on other medications, may respond differently. If your vet recommends firocoxib beyond 14 days, periodic bloodwork to check kidney and liver function is a reasonable precaution.
Competition Rules
Firocoxib is a permitted substance under United States Equestrian Federation (USEF) rules, but with strict conditions. The maximum permitted plasma concentration is 0.240 micrograms per milliliter. No part of a dose can be given within 12 hours of competing, and any medicated feed must be consumed or removed at least 12 hours before competition. The maximum daily dose allowed is 0.0455 mg per pound of body weight (about 45.5 mg for a 1,000-pound horse), and the maximum treatment duration recognized by USEF is 14 consecutive days.
If you compete under FEI rules, the detection times and thresholds may differ. FEI publishes its own detection time list, and you should check it separately before competing internationally.