Medroxyprogesterone acetate, also known as medroxy or MPA, is a compounded version of Depo-Provera, a human contraceptive. This drug has been used for years in the sport horse industry, believed to calm fractious horses and prevent estrus in mares (though research shows it actually has no impact on estrus). As a calming medication, medroxy is believed to work on the GABA receptors in the brain, similar to Xanax and Valium. As a compounded drug, medroxy isn't approved by the U.S. Food and Drug Administration for use in animals, reports Chronicle of the Horse.
If medroxy is delivered intravenously (rather than intramuscularly), the horse can die within minutes as the medication rushes to the horse's brain. However, even when delivered correctly, in the muscle, there is a risk of allergic reaction—and death.
Though there have been no official warnings by the FDA about the use of medroxy, sudden equine death from medroxy injection is not totally uncommon, according to The Chronicle. Some veterinarians told the magazine they refuse to prescribe or administer the compounded drug and others ask clients to sign a disclosure whenever medroxy is requested.
If a horse has a reaction to a compounded drug, the owner or veterinarian must report it to the pharmacy that made the drug. Once the pharmacy is made aware however, they are not required to report deaths to a state licensing board or the FDA. For this reason, there is no cohesive data collection on how many horses are dying from medroxy injections.
Equestrian Canada and the Fédération Equestre Internationale (FEI), which oversees international horse sport, have both banned the use of the drug in competition horses. The U.S. Equestrian Federation still allows the use of the drug.
Read more at Chronicle of the Horse.
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