Progesterone is a steroid hormone produced by the mare in pregnancy (by the ovary in early pregnancy, and by the fetus and placenta in later pregnancy). Progesterone is required for pregnancy. Progesterone has an inhibitory effect on the estrus cycle in mares and reduces estrus behavior.

Altrenogest (Regu-Mate®, Altresyn®) is a liquid, synthetic, oral progestin that is used to suppress estrus in mares. It is used to facilitate scheduled breeding, to maintain pregnancy, and to suppress undesirable behaviors (“hormonal distractions”) resulting from estrus. Altrenogest has a direct effect on the uterine muscle, causing a variety of effects typical of progesterone, the natural hormone of pregnancy in the mare.

YOUR VET'S ROLE

Your vet may recommend this medication for suppression of estrus in the mare for a variety of reasons, mostly behavioral. They may recommend it to support and maintain pregnancy, especially in mares that have been tested and have low blood progesterone. In rare cases, it may be suggested in male horses, also for behavioral modification.

YOUR ROLE

Talk to your vet about whether this treatment is appropriate for your horse, and for you. It is fed daily to mares in their feed, or orally dosed daily per your vet’s recommendations. Regumate is given commonly and without incident, but it does have a reputation for causing a variety of potential side effects in humans coming into contact with it. Please see contraindications and side effects, and determine whether you are at risk, before potentially exposing yourself to it.

Antrenogest is contained in a very oily, absorptive base and requires the use of rubber (nitrile) gloves when handling. It should not be handled by pregnant women, or women who believe they may be pregnant.