Position on the Use of Medroxyprogesterone Acetate in Competition Horses

Medroxyprogesterone acetate (MPA) is a synthetic progestin hormone administered to mares off-label in an attempt to suppress behavioral estrus. However, a controlled research study found that MPA was not effective at suppression of behavioral estrus.¹ Many veterinarians believe MPA modifies behavior by producing a calming effect in the horse and does not have a therapeutic benefit that goes beyond this behavior modification. Therefore, the AAEP recommends that MPA should not be administered to horses in competition.

1. Gee EK, C DeLuca, JL Stylski, PM McCue.  Efficacy of medroxyprogesterone acetate in suppression of estrus in cycling mares.  J Equine Vet Sci 2009;29:140-145.

Reviewed by the AAEP board of directors in 2025.