Research on Estrus Suppression Drug

Medroxyprogesterone (MPA) has been used in an attempt to suppress estrous behavior, but it lacks therapeutic efficacy.
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ears back mare field

Medroxyprogesterone (MPA) has been used off label in mares as an aid in suppressing estrus and estrous behavior. It turns out that it does not accomplish this and, in fact, has been associated with cases of anaphylaxis.

At the 2019 AAEP Convention, the veterinarian audience was reminded about a recent update to the prohibited substances list. Medroxyprogesterone (MPA) has been used off label in mares as an aid in suppressing estrus and estrous behavior. It turns out that it does not accomplish this and, in fact, has been associated with cases of anaphylaxis. There were 23 fatalities reported over a three-year period.

Due to lack of therapeutic efficacy as well as significant safety concerns of MPA use in horses, as of December 31, 2019, the United States Equestrian Federation (USEF) prohibited its use in USEF-licensed competition horses. This ban will be strictly enforced starting June 1, 2020, based on a positive drug test. 

The lag time between the ban and enforcement is due to the length of time the drug takes to clear a horse’s system. 

Penalties will include a 3- to 6-month suspension from competition and a first-offense fine of $3,000 to $6000.

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