Altrenogest, which is commercially sold as Regu-Mate, is an FDA-approved synthetic progesterone used to suppress estrus and seasonal behaviors in competing horses and aid in breeding practices with broodmares. Altrenogest, or any other progestogen, is also prohibited for use in stallions and geldings while competing in USEF-licensed competitions because it has no legitimate therapeutic use in male horses and has been historically used as a calming agent.
The prohibition of altenogest, or any progesterone, in stallions and geldings went into effect December 1, 2024, and for the 2025 season, anyone with a positive drug test received a warning. However, beginning in the 2026 competition season, positive tests may now prompt penalties.
Under the penalty guidelines, altrenogest is classified as a Category II.a. substance, which encompasses substances that have legitimate therapeutic uses in horses but are prohibited to use in USEF-licensed competitions. In this case, this only applies to stallions and geldings, as mares are permitted to receive altrenogest while competing. Because it’s routinely in the environment, altrenogest positives receive a lesser penalty than other Category II substances.
“We have had findings that have been adjudicated, and we do recognize that when altrenogest is in the barn for mares, there’s potential for inadvertent exposure or administration,” said Dr. Stephen Schumacher, Chief Veterinary Officer at US Equestrian. “Typically, what happens is the wrong feed buckets are used, or the stallion or gelding is put in the mare’s stall where there’s unconsumed food.”
To minimize the risk of potential exposure or inadvertent administration, it’s important to establish strict barn practices including using a separate, clean bucket for each horse and removing buckets from the stall after feeding time. Another good practice is to feed mares receiving altrenogest in a designated group either at the beginning or the end of feeding time.
Anyone who uses altrenogest should also be aware of the potential for adverse reactions in humans, particularly in women of child-bearing age. Because altrenogest is synthetic and oil-based, it will readily absorb through the skin, so vinyl, neoprene, or nitrile gloves should be worn when handling the product or anything that comes into direct contact with it.
While Regu-Mate is the brand name most people are familiar with, there are also two FDA-approved generic equivalents available on the market, Altren and Matrix. Although some people use injectable altrenogest, it’s important to know that none of these options are FDA-approved. The injectables source of altrenogest are compounded, and there can be some risk with using a compounded product, as compounding pharmacies are required to make medications in small batches as prescribed, so there can be significant concentration variations in one batch to the other. There can also be significant differences in internal controls at those pharmacies, so you should work with your veterinarian to find one with good quality controls and a track record of staying compliant with state and federal laws.
Using altrenogest is a viable option to help normalize mares during their cycles, but care should be taken when using it to minimize potential exposure in stallions and geldings, which can lead to an inadvertent positive on a drug test.