Horse Side Vet Guide ®

Equine Health Resource

Altrenogest, Oral Progesterone



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). It 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.

Talk to your vet about whether this treatment is appropriate for your horse. It is fed daily to mares in their feed, or orally dosed daily per your vet’s recommendations.

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

Consider Potential Side Effects & Complications

Altrenogest can perpetuate a uterine infection. It can also cause the cervix to close, which may decrease uterine defenses.

Consider Reasons Not To Use This Treatment

Not used in mares with a uterine infection because it may worsen it. It should not be handled by pregnant women, or women who believe they may be pregnant.

Skills I might need

Is It working? Timeframe for effect

Within a day or two of administration, you may notice behavioral changes in your mare. The actual effects on the uterus probably take place within hours.

Questions To Ask My Vet

  • What precautions do I need to take in handling altrenogest?

Helpful Outside ResourcesCredible Equine Health Information on the Internet

Ceva Animal Health Inc. Altresyn® Solution, 0.22% (altrenogest)
Merck Animal Health Regu-Mate® Solution (altrenogest)
* Would you like to showcase your equine product or service in
Horse Side Vet Guide? Contact us to start the conversation.
Author: Doug Thal DVM Dipl. ABVP


We're not around right now. But you can send us an email and we'll get back to you, asap.