The United States Equestrian Federation Board of Directors has voted to prohibit the use of medroxyprogesterone acetate (MFA) which is the generic version of Depo-Provera —  in horses competing in USEF-licensed competitions effective Dec. 1. The USEF has been considering a ban on MPA since 2017, driving mostly issues with anaphylaxis, but also because of its use as a calming agent (more on that below). According to the press release,

The Panel determined MPA has no therapeutic use in competition horses, as it does not interrupt estrus in mares, which predicated its original use. Additionally, MPA is not approved by the United States Food and Drug Administration (FDA) for use in equines and its use has been reported and documented to be associated with several cases of anaphylaxis and fatality. As a result of this analysis, the Panel voted unanimously to recommend MPA is added to the list of USEF prohibited substances.

MPA first began to be used in horses 30 years ago as an alternative to Regu-mate (altrenogest), which is used to control heat cycles in mares, but carries significant health risks for the human women who have to handle and administer it to horses orally. However, MPA’s effect has never been studied in horses and dosing amounts never established. For the most part, the benefits of MPA are anecdotal.

Technically, the human drug, Depo-Provera, is not approved for use in horses, so while people speak about a horse being on Depo, it’s actually a compound of MPA. The human drug was developed as an injectable contraceptive; in horses, it was originally prescribed to reduce “mare-ish” behavior, but its use has become more widespread. MPA is routinely prescribed for attitude adjustments regardless of gender.

Trainers say that MPA helps a horse “focus” and become more manageable. So, in some circles, MPA is used to treat horses that might be irritable, difficult under saddle, difficult to handle, etc. Some people believe that MPA may mask underlying unsoundness or discomfort, which causes behavioral issues in horses.

According to Dr. Stephen Schumacher, DVM, Chief Administrator, USEF Drugs and Medications Program, the perceived calming effect that MPA reportedly has on mares, geldings and stallions may be a result of a reaction with GABA receptors to create an effect similar to tranquilizers. This has been studied and proven in rats, but not yet in horses (Focus on Depo-Provera and Horse Showing).

Proponents of MPA, like trainer Mary Babick, who is United States Hunter Jumper Association President (USHJA), speak of a “focusing effect” that makes horses easier to train, and a stress-relieving effect for horses in a very unnatural competitive environment. Many people believe that allowing horses to show on MPA makes showing safer.

Personally, I’m not sure I buy that. Given that it is against the rules to give a horse a performance-enhancing supplement to show, I find it hard to believe that MPA isn’t being used in this way. The USEF states:

Any product is prohibited if it contains an ingredient that is a prohibited substance, or is a drug which might affect the performance of a horse and/or pony as a stimulant, depressant, tranquilizer, analgesic, local anesthetic, psychotropic (mood and/or behavior altering) substance, or might interfere with drug testing procedures.

I’m quite appalled by how common it is now to use drugs to control the moods of horses for showing. Granted, I never competed at big rated shows; mostly we toughed it out with the horses we rode and sometimes it wasn’t pretty. I don’t remember anyone offering us a chemical solution to an unruly or anxious horse. We might lunge a bit before riding, but mostly we were told to “ride better.”

I’m sure there are some horses that need hormonal regulation. Regu-mate is not a drug that most people want to handle. According to the FDA it can cause adverse reproductive effects in women and girls including abnormal or absent menstrual cycles. Men can experience decreased libido. Both sexes may suffer from headaches, fever, abdominal pain, nausea, diarrhea, vomiting and rashes. No thank you.

Thank goodness Zelda has never been “mare-ish” or become difficult when in heat. But even if she had, I don’t think I’d be reaching for MPA.

What has your experience been with Depo?