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). 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 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.


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.

Consider Potential Side Effects & Complications

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

Altrenogest may cause cancer and heart disease. If you spill it on yourself, you should let your doctor or gynecologist know.

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. In post-menopausal women, it may be associated with increased risk of cancer and cardiovascular disease.

Altrenogest should not be handled by pregnant women, anyone with a history of blood clot, hardening of the arteries, or stroke, women with history of breast cancer, or estrogen-dependent cancer, women with undiagnosed vaginal bleeding, women who have had cancer develop while taking estrogens or oral contraceptives. (from Plumb- Veterinary Drug Handbook (Client Information Edition- c2003)

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?
  • Is this drug safe for me to administer to my horse?

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.