Isoxsuprine for Veterinary Use

Therapeutic Class: Peripheral vasodilator

Species: Horses

Indications: Navicular disease

Isoxsuprine is a mixed beta-adrenoreceptor antagonist/agonist. It causes relaxation of vascular and uterine smooth muscle. In addition to peripheral vasodilation, isoxsuprine has been shown to decrease blood viscosity and platelet adherence. Isoxsuprine is primarily used in veterinary medicine to treat or manage navicular disease in the horse. It is used in human medicine as a tocolytic agent to decrease preterm labor or abortion. The human gastrointestinal tract absorbs isoxsuprine well, but studies in the horse indicate that oral bioavailability is approximately 2.2%.

Isoxsuprine is used in horses for the management of navicular disease and laminitis. Due to the low bioavailability in the horse, treatment with oral isoxsuprine is not universally accepted, but there are veterinarians who feel it is beneficial in some cases.

Because of its demonstrated tocolytic properties in humans, some veterinarians have used isoxsuprine for the same purpose in broodmares. Its efficacy for this use has not been well-evaluated.

• Side effects after oral administration are rare in the horse because of the low level of absorption. Possible side effects could include low blood pressure, increased heart rate, and possible gastrointestinal irritation.

• Side effects after intravenous administration include hypotension, tachycardia, CNS stimulation, and sweating.

• Isoxsuprine is a vasodilator and should not be used in mares immediately after foaling or horses that are actively bleeding.

• Despite the low level of oral absorption, isoxsuprine can be found in a horse’s urine for many weeks after treatment is discontinued. This may be a consideration for competition horses.

• There are no safety studies on isoxsuprine use in pregnant or lactating mares. There are no safety studies regarding the effects on semen in breeding stallions.

• Drug interactions are unlikely with oral isoxsuprine.

• Overdose of isoxsuprine increases the risk and severity of the above mentioned side effects.