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Regu-Mate Side Effects

| 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.


  • Ask your vet about trying other remedies for your moody mare before resorting to Regu-Mate, including herbal ones such as raspberry leaves. Not only are these available over the counter, but they are safe for female barn help to administer.

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.


  • Pregnant women should not handle Regu-Mate, nor should women who have been diagnosed with breast or reproductive cancers. In general, women of child-bearing age should take precautions when handling Regu-Mate. Anyone who has heart disease or has suffered a stroke or embolism should not handle this drug. When administering Regu-Mate, wear at least two layers of disposable gloves, protective glasses and coveralls. Wash your hands thoroughly after administering the drug.