Isoxsuprine is a drug that enlarges the arteries (vasodilator). Despite a dearth of supportive information on its efficacy it has come to be used widely in equine medicine for nonspecific lameness, navicular disease (syndrome), and laminitis. This article discusses the scientific evidence for its use in the horse, the contraindications, toxic effects, dosages, and association drug rules that apply to this medication.

There is no clear work to show this is an efficacious drug for lameness in the horse and several studies that cannot find a pharmacological effect at nontoxic doses, (Absence of detectable pharmacological effects after oral administration of isoxsuprine.; Equine Vet J 1998 Jul;30(4):294-9; Harkins JD, Mundy GD, Stanley S, Woods WE, Sams RA, Richardson DR, Grambow SC, Tobin T; Maxwell H. Gluck Equine Research Center and the Department of Veterinary Science, University of Kentucky, Lexington 40506, USA.) . There has been a study that found a benefit in early navicular disease (syndrome), but until a mode of action can be found this finding must remain questionable and it's use considered experimental.

J Vet Pharmacol Ther 2002 Apr;25(2):81-7 Isoxsuprine hydrochloride in the horse: a review. Erkert RS, Macallister CG. Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Oklahoma State University, Stillwater, OK, USA. Isoxsuprine hydrochloride has been suggested for use in horses for treatment of navicular syndrome and laminitis. The drug has been shown to be a beta-adrenoreceptor antagonist with beta-adrenoreceptor agonistic properties, with both characteristics contributing to vasodilation and uterine relaxation. In addition, the drug is capable of decreasing blood viscosity and platelet aggregation. Studies have shown i.v. isoxsuprine to have a plasma half-life of <3 h with a large apparent volume of distribution. Cardiovascular effects resolve rapidly following i.v. administration, but are absent with oral dosing. Oral bioavailability is 2.2% with a high first pass effect. Isoxsuprine has an apparent affinity for melanin that may contribute to extended renal excretion. Clinical trials appear to support the use of isoxsuprine for treatment of navicular disease. However, poor bioavailability, lack of cardiovascular effects following oral administration, superficial support in clinical trials, and new evidence regarding the pathogenesis of navicular syndrome indicate that the use of isoxsuprine for treatment of navicular syndrome or laminitis is questionable at best.