Altrenogest is one of the most commonly prescribed hormonal medications in equine reproductive management. If your veterinarian has mentioned it for your mare, or you're comparing it against Regu-Mate and other options, here's what altrenogest is, how it's used, and what handlers need to know before starting it.
Altrenogest is a synthetic progestin, meaning it mimics the activity of the natural hormone progesterone. Veterinarians prescribe it primarily to suppress or synchronize the estrous cycle in mares.
Definition: A progestin is a synthetic compound that produces effects similar to progesterone, the hormone that naturally rises during a mare's diestrus phase and helps regulate the reproductive cycle.
Altrenogest at a Glance |
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Attribute: |
Detail: |
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Drug class |
Synthetic progestin (19-nortestosterone derivative) |
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Common brand name |
Regu-Mate® |
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Route |
Oral (compounded injectable forms also available) |
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Standard labeled dose |
0.044 mg/kg (about 1 mL per 110 lbs body weight) once daily |
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Typical course length |
15 consecutive days |
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Prescription status |
Prescription only, through a licensed veterinarian |
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Available at Mixlab as |
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Note: Dosing shown reflects the labeled reference dose for Regu-Mate; your veterinarian will determine the appropriate dose, form and course length for your mare.
Veterinarians most often prescribe altrenogest to suppress estrus, which helps synchronize breeding schedules, and to address behavioral signs associated with estrus.
Mares in estrus can show irritability, vocalization, frequent urination, "teasing" behavior toward other horses and general mood changes. When a veterinarian determines estrus is the driver of these behaviors, altrenogest may be part of the management plan alongside other considerations specific to the mare.
Our Take: We hear from equine veterinarians that altrenogest gets requested as a blanket "calm my mare down" solution more often than its label supports. It's specifically indicated for estrus-related behavior, not general anxiety or training issues, so ruling out estrus as the cause is a necessary first step, not an afterthought.
For more on how the underlying cycle works, see our guide on What Is the Estrous Cycle? →
The labeled dose for altrenogest is 0.044 mg/kg administered orally once daily for 15 consecutive days, typically given directly on the tongue or mixed into grain. According to Merck Animal Health's Regu-Mate product page, this works out to approximately 1 mL per 110 pounds of body weight per day.
Compounding allows for adjustments to strength, flavor or delivery form when a standard commercial dose doesn't fit a mare's needs or a barn's routine. This is where a pharmacy like Mixlab comes in.
For the full administration protocol, including handling precautions during dosing, see our detailed guide: The Administration of Altrenogest in Mares →
Have a prescription in hand? Order compounded altrenogest through Mixlab's equine pharmacy for two-day delivery.
For horses, altrenogest is used routinely in equine veterinary practice, and your veterinarian will discuss any horse-specific precautions, including avoiding use in mares with uterine inflammation.
For the people handling it, altrenogest carries a well-documented human safety warning that's worth taking seriously.
According to the FDA, altrenogest exposure was linked to 130 reported cases affecting 137 people between 1987 and 2018, most of them women. Reported effects included menstrual cycle disruption in women and decreased libido in some men. The agency requires impermeable gloves (nitrile, butyl, vinyl or neoprene, not latex) for anyone handling the product, and pregnant women or women who may be pregnant should not handle it at all.
Why does this matter for a blog about a horse's medication? Because altrenogest is absorbed through unbroken human skin, which makes handling technique part of "using it safely," not just dosing the horse correctly.
Altrenogest isn't the only tool veterinarians use for reproductive management, and it isn't always the right fit on its own.
Altrenogest + Alternative Medications |
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Option: |
Type: |
Common Role: |
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Altrenogest |
Oral progestin |
Suppresses estrous behavior; may not fully suppress ovulation |
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Prostaglandins |
Injectable hormone |
Shortens diestrus to help synchronize cycles |
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hCG (human chorionic gonadotropin) |
Injectable hormone |
Supports timing of ovulation around breeding |
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Progesterone/estrogen combinations |
Injectable hormone |
Used for estrus suppression and synchronization protocols |
Your veterinarian may combine two or more of these depending on your mare's cycle history, breeding goals and behavior pattern.
Altrenogest is not only used for breeding. It's used far more broadly than that. Behavioral management during estrus is one of the most common reasons it's prescribed, independent of any breeding plan.
No, altrenogest doesn’t necessarily stop ovulation. Altrenogest and other oral progestogens suppress estrous behavior, but they may not effectively suppress follicle growth and ovulation in cyclic mares. That distinction matters for anyone assuming the medication alone prevents pregnancy risk.
Altrenogest is available commercially (as Regu-Mate) and through compounding pharmacies like Mixlab, which can adjust strength or delivery form. The active ingredient and labeled use are the same; the format your veterinarian prescribes depends on your mare's needs.
Regu-Mate is the commercial brand name for altrenogest. Compounding pharmacies like Mixlab can also formulate altrenogest outside the commercial product when a veterinarian prescribes a different strength or form.
The labeled reference dose is 0.044 mg/kg given orally once daily for 15 consecutive days, though your veterinarian sets the exact dose and course for your mare.
Reported adverse effects in horses from altrenogest are uncommon when used as directed. Your veterinarian can walk through what to watch for based on your mare's health history.
Altrenogest can be dangerous for humans to handle if handled improperly. The FDA has documented over 130 human exposure cases tied to menstrual disruption and other effects, so impermeable gloves and careful handling are required, not optional.
Not reliably on its own. It suppresses estrous behavior, but per the Merck Veterinary Manual, follicle growth and ovulation may still occur in some mares.
Yes, altrenogest can be compounded. Mixlab compounds altrenogest to specific strengths and delivery forms when a commercial dose doesn't fit a mare's needs, prescribed by your veterinarian.
Altrenogest remains one of the most widely prescribed tools for managing estrus-related behavior and breeding timing in mares. It's typically prescribed as part of a plan your veterinarian builds around your mare's specific cycle and health history, and it deserves the same handling care you'd give any hormone medication.
If your veterinarian prescribes altrenogest, Mixlab can fill it in commercial or compounded formulations and deliver it directly to your barn.
Looking to order altrenogest for horses? Mixlab fills commercial and compounded formulations with two-day delivery.