Altrenogest for Horses: Uses, Dosing & Side Effects

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Altrenogest is a synthetic progestin, meaning it acts like the natural hormone progesterone. In horses, it is most commonly used in mares to suppress visible heat behavior and help your vet manage the reproductive cycle more predictably. The best-known FDA-approved equine product is Regu-Mate® (altrenogest) Solution 0.22%, an oral liquid containing 2.2 mg/mL.

In practical terms, this medication is often used when estrus behavior is interfering with training, showing, handling, or breeding plans. It does not fix every behavior problem, and it is not a substitute for a reproductive exam. If a mare has pain, ovarian disease, uterine disease, or another medical issue, your vet may recommend testing before deciding whether altrenogest makes sense.

This is also a medication with important human safety concerns. Altrenogest can be absorbed through unbroken skin. Pregnant women, women who may be pregnant, and people with certain hormone-sensitive or clotting-related health risks should not handle it. Your vet may recommend gloves, careful dosing technique, and avoiding contaminated surfaces or equipment.

The labeled use of altrenogest in horses is to suppress estrus in mares. Merck Veterinary Manual notes that progestogens such as altrenogest are commonly used when estrous behavior is undesirable in performance mares, and the FDA-approved label states it is indicated to suppress estrus. After the medication is stopped, many mares return to estrus in a more predictable window, which can help with breeding management.

Your vet may also use altrenogest extralabel in selected reproductive cases, such as short-term cycle control or pregnancy support in mares your vet considers at risk. That decision should be individualized. Not every mare with behavior changes needs hormone therapy, and not every pregnant mare benefits from supplementation. A reproductive exam, ultrasound, and sometimes hormone testing help your vet decide whether treatment is appropriate.

Because behavior changes can overlap with pain, ulcers, back soreness, ovarian tumors, or uterine disease, altrenogest should be viewed as one option, not the only answer. If a mare suddenly becomes reactive, uncomfortable under saddle, or difficult to handle, your vet may want to rule out medical causes before starting long-term hormone therapy.

For the FDA-approved indication of estrus suppression, the labeled dose is 1 mL per 110 lb body weight by mouth once daily, which equals 0.044 mg/kg/day. In a 1,000 lb mare, that works out to about 9.1 mL once daily. Merck Veterinary Manual also lists 0.044 mg/kg by mouth daily for estrus suppression. The medication may be given directly by dosing syringe or applied to feed if your vet says that is appropriate.

For many mares being managed for estrus suppression, treatment is given for 15 consecutive days. Your vet may choose a different schedule for an extralabel reproductive reason, but that should only be done with veterinary guidance. If a dose is missed, do not double the next dose unless your vet specifically tells you to. Consistency matters with hormone medications, so it helps to give it at the same time each day.

Before starting altrenogest, your vet may recommend a reproductive exam because the product is contraindicated in mares with previous or current uterine inflammation, including endometritis. Your vet should also review whether the mare is intended for human consumption, because labeled products state do not use in horses intended for human consumption.

Handling matters as much as dosing. Wear vinyl, nitrile, or neoprene gloves, avoid skin contact, and clean any contaminated syringes or surfaces promptly. Latex gloves are not considered protective on the current product safety information.

Many mares tolerate altrenogest well when it is used correctly, but side effects and treatment failures can still happen. The manufacturer notes that about 5% of mares may continue to show estrus despite treatment. That does not always mean the medication was given incorrectly. Some mares need a recheck to look for ovarian activity, uterine disease, or another reason the response is incomplete.

In the horse, your vet may ask you to watch for changes in attitude, appetite, swelling, discharge, or signs that the mare is still cycling. If altrenogest is being used in a reproductive case rather than routine estrus suppression, your vet may also monitor with ultrasound or bloodwork depending on the goal of treatment.

The biggest safety concern is often human exposure, not the mare. Accidental skin exposure can cause menstrual cycle disruption, abdominal or uterine cramping, headache, and possible prolongation of pregnancy in people. Wash spills off skin right away with soap and water, and contact your physician if exposure occurs and symptoms develop.

Call your vet promptly if your mare seems painful, develops abnormal vaginal discharge, spikes a fever, shows worsening behavior, or does not respond as expected. Those signs can point to an underlying problem that needs a different plan.

There are no widely emphasized routine drug interactions listed on the standard equine product information in the way you might see with some antibiotics or heart medications. Still, hormone therapy should never be treated as interaction-free. Your vet needs a full list of everything your horse receives, including reproductive hormones, joint injections, ulcer medications, supplements, and compounded products.

The most important practical concern is often therapeutic overlap rather than a classic drug-drug interaction. For example, if your mare is also receiving other reproductive hormones or treatments intended to manipulate the cycle, your vet will want to coordinate timing carefully. Using multiple hormone strategies without a plan can make cycle interpretation harder and may increase the chance of poor results.

Altrenogest also should not be used casually in mares with suspected uterine infection or inflammation, because progestin therapy can worsen a smoldering uterine problem. That is technically a contraindication rather than an interaction, but it is one of the most important safety points to discuss with your vet.

If your mare is pregnant, breeding, showing, or competing, tell your vet and trainer before starting treatment. Competition rules, withdrawal guidance, and reproductive plans may affect whether altrenogest is the right option and how it should be timed.