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Regu-Mate Side Effects

By Jane Meggitt | Updated September 26, 2017

In the horse world, describing a female equine as "mareish" isn't a compliment. It means that she's difficult during her heat cycles, becoming distracted and possibly somewhat squirrely. That's where Regu-Mate can help.

Regu-Mate Uses

Regu-Mate is the trade name for altrenogest, a synthetic hormone. Regu-Mate can normalize a mare's troublesome behavior, as it suppresses her heat cycle. It's also given on a schedule to breeding mares to regulate their estrous cycles, and to help mares with a history of breeding difficulties sustain early pregnancies. If a mare is scheduled for an embryo transfer, Regu-Mate can synchronize her cycle with that of the mare who will carry the foal. The drug is only available via veterinary prescription.

Regu-Mate Administration

Your vet will prescribe the right Regu-Mate dosage for your mare. Administer Regu-Mate by drawing up the correct amount in the syringe, then either placing it on the base of animal's tongue or by mixing in her grain ration.

Side Effects and Contraindications

Although there are few side effects when given to mares, it can affect fillies born to mares receiving the drug. These genitalia of these fillies is somewhat masculinized, with larger than average clitoral size. Fillies whose mothers received Regu-Mate experienced their first heat cycle earlier than fillies whose mother did not receive the drug. However, reproductive outcome is similar in broodmares whose mothers received Regu-Mate and those who did not.

Mares diagnosed with endometritis, or uterine inflammation, should not receive Regu-Mate. Approximately 5 percent of mares receiving Regu-Mate will not respond to the drug and continue to have regular heat cycles.


Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.