Isoxsuprine (Book Excerpt)

Generic Name: Isoxsuprine hydrochloride
Common Brand Name: Generics
Drug Type: Vasodilator
Indications: Navicular disease, laminitis

Basic Information
Isoxsuprine is most frequently used in horses for the management of navicular disease and laminitis although it is not a universally accepted treatment. Some veterinarians believe it helps, and others remain skeptical. This is partly because there has been conflicting research on the effects of isoxsuprine and partly because the underlying causes of navicular disease and laminitis are not completely understood.

Isoxsuprine is a vasodilator; it works by relaxing the smooth muscle around peripheral blood vessels. Impaired circulation and blood flow are probably contributing causes of both navicular disease and laminitis. Circulatory changes in the foot during laminitis are currently an area of intense research. As these diseases become better understood, improved methods of prevention and treatment are likely to evolve.

Side Effects, Precautions, and Overdose

  • Side effects after oral administration are rare in the horse but can include changes in blood pressure, increased heart rate, and possible gastrointestinal irritation. In humans dizziness, weakness, and other central nervous system signs are also reported.
  • Isoxsuprine is a vasodilator and should not be used in mares immediately after foaling or horses that are actively bleeding.
  • Overdose of isoxsuprine increases the risk and severity of the aforementioned side effects.

Drug Interactions

  • Isoxsuprine should be used with caution with other drugs that might affect blood pressure. That includes most sedatives and drugs used for general anesthesia. It is important to keep accurate records of isoxsuprine and any other medications used if an animal is referred to an equine hospital for intensive care or surgery.

Special Considerations

  • In humans isoxsuprine is sometimes used in the treatment and management of premature labor. Some veterinarians have used isoxsuprine for the same purpose in broodmares, but its efficacy has not been scientifically established.
  • Isoxsuprine is not FDA approved in the horse. It is commonly used and an accepted practice, and it is a prescription drug. U.S. federal law restricts this drug to use by or on the lawful written or oral order of a licensed veterinarian within the context of a valid veterinarian-client-patient relationship.

Special Populations
Breeding Animals
There are no safety studies on isoxsuprine use in pregnant or lactating mares. There are no safety studies on effects on semen in breeding stallions.

It is unlikely that isoxsuprine would be used in foals, and no information was found on this use.

Pony breeds do not appear to differ from horses in their response to isoxsuprine.

No information was found on isoxsuprine use in older horses. Isoxsuprine is metabolized by the liver. Liver function should be checked before administering isoxsuprine.

Competition Horses
Isoxsuprine is prohibited in any drug-free competition. It is permitted in some types of USEF competition and not in others. It is a prohibited class B medication under the new FEI rules. Oral drugs are much more likely to have variable detection times, and long-term or repeated doses can also affect detection times for many drugs. Detection time for isoxsuprine in urine is several weeks to months if the more sensitive ELISA test is used. It is important to consult with a knowledgeable veterinarian and the individual regulatory group.

Dose and Route of Administration
Oral: 0.1 to 0.3 mg/lb starting twice a day. This may be decreased to once a day. There are other dosing regimens.
Dose Form: 20 mg tablets

About the Author

Barbara D. Forney, MS, VMD

Barbara D. Forney, MS, VMD, author of Understanding Medications for Horses

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