Equine Pain Relief Tool Kit: Veterinarians Add Acetaminophen

Nonsteroidal anti-inflammatory drugs (NSAIDs) continue to monopolize the pain management spotlight despite their inherent and clinically important disadvantages. Veterinarians can prescribe acetaminophen either as an adjunct or standalone medication to help horses suffering from painful conditions, particularly lameness.

Although effective and familiar to veterinarians, NSAIDs are associated with several side effects, including gastric ulcers and right dorsal colitis, which is inflammation of a specific region of the large colon. These side effects occur because, in addition to blocking cyclooxygenase enzymes involved in inflammation, this class of drug also blocks the beneficial cyclooxygenase enzymes that keep gastrointestinal tract tissues healthy.

“On the contrary, acetaminophen has fewer effects on the gastrointestinal tract, as it works by different mechanisms than NSAIDs. Together with its wide margin of safety, acetaminophen can either be combined with NSAIDs or used as a standalone medication for pain relief,” explained Kathleen Crandell, Ph.D., a Kentucky Equine Research nutritionist.

Studies showing safety of acetaminophen have been published, and doses of 25 mg/kg twice daily for up to 30 days had no negative effects on either liver or kidney function. Further, a dose of 20 mg/kg appeared as effective as the NSAID flunixin meglumine at a standard dose of 1.1. mg/kg at improving lameness.

In a recently published study, nine healthy adult mares had lameness induced by applying a modified horseshoe that placed pressure over four areas of the sole.^* Horses were treated with either 20 mg/kg or 30 mg/kg acetaminophen, 2.2 mg/kg phenylbutazone (an NSAID commonly referred to as “bute”), or a placebo. All horses received each treatment with a three-day washout period between each. The acetaminophen formulation used in this study was extra-strength Tylenol (500-mg caplets).

Peak blood levels for acetaminophen were 30.02 mg/mL blood at 0.43 hr post-administration after 30 mg/kg dosing, and 20.01 mg/mL blood 0.66 hr post-administration of 20 mg/kg..

In addition to being absorbed faster and achieving higher circulating levels in the bloodstream, horses receiving 30 mg/kg acetaminophen also had significantly improved lameness scores compared to the control group at both two and four hours following administration. Even the phenylbutazone group only showed an improvement in lameness eight hours after oral administration.

“These results suggest that 30 mg/kg acetaminophen may be suitable not only as monotherapy for lameness, but it can also be used in conjunction with NSAIDs for pain management,” explained Crandell.

The researchers did note, however, that additional studies evaluating peak absorption and assessment of side effects are needed at the twice daily dosing interval with and without NSAIDs.

Horse owners should continue to employ proactive measures to reduce the incidence of certain lameness, such as high-quality joint supplements, Crandell suggested.

 ^*Mercer, M.A., H.C. McKenzie, C.R. Byron, R.S. Pleasant, S.H. Boges, R.M. Council-Troche, S.R. Were, T. Burns, and J.L. Davis. 2022. Pharmacokinetics and clinical efficacy of acetaminophen (paracetamol) in adult horses with mechanically induced lameness. Equine Veterinary Journal:13601.