Mare are seasonally polyestrus, which means they have an anovulatory period during the short light days of late fall and early winter, and begin to ovulate as the days become longer beginning in the spring. The sexual behavior of the mare probably evolved in the social context of family groups that typically consist of multiple adult mares and their young offspring, plus one or more adult stallions.1 Behavior that identifies mares as being estrous or non-estrous status has been grouped into three categories: attractivity, proceptivity, and receptivity.2

During the ovulatory season, the developing follicle releases estradiol (an estrogen), which brings on estrous (or “heat”) behaviors. Nature’s purpose for this dynamic is to facilitate the mare attracting a stallion. Unfortunately, these behaviors—frequent urination, squealing, tail swishing, and threatening other horses—are the ones that hamper a mare’s trainability and performance. Estrogen also results in muscle relaxation, so it’s possible that subtle lameness issues become more uncomfortable for when muscular support is lacking, leaving the mare more at risk for soft-tissue strains. Mares can also become quite uncomfortable as follicles develop—to the point of presenting with full-blown colic symptoms during ovulation.1

Medroxyprogesterone for Horses

Medroxyprogesterone (medroxyprogesterone acetate, MPA, Provera®, Depo-SubQ provera 104®, Depo-Provera®) is a synthetic progestin used primarily to treat sexually dimorphic behavior problems such as roaming, inter-male aggressive behaviors, spraying, mounting; sometimes used to treat feline psychogenic dermatitis and alopecia X in dogs. Progestins are primarily produced endogenously by the corpus luteum. They transform proliferative endometrium to secretory endometrium, enhance myometrium hypertrophy, and inhibit spontaneous uterine contraction. Progestins have a dose-dependent inhibitory effect on the secretion of pituitary gonadotropins and can have an anti-insulin effect. Medroxyprogesterone has exhibited a pronounced adrenocorticoid effect in animals (species not listed) and can suppress ACTH and cortisol release.3

Progesterones have been used in horses for many purposes, including management of the spring transition period, prevention of estrus behavior, induction of estrous cycle synchrony, pregnancy maintenance, and modification of stallion behavior.3 However, medroxyprogesterone does not appear to effectively suppress estrous behavior or follicular activity in normal cycling mares.4 Medroxyprogesterone (Depo-Provera®) is usually dosed at ≈500 – 800 mg IM. The interval between shots varies between horses. Most injections last 2-3 months. This drug will not prevent pregnancy loss and does not stop cyclicity. However, MPA does not appear to effectively suppress estrous behavior or follicular activity in normal cycling mares.3

In 2019, the United States Equestrian Federation (USEF) voted to prohibit the use of Medroxyprogesterone Acetate (MPA) in horses competing at licensed competitions, citing a lack of therapeutic use in competition horses and the fact that MPA is not approved by the U.S. Food and Drug Administration (FDA) for equine use. MPA is now classified as a Category III substance by the USEF and carries fines and penalties for its use.

Where to buy Medroxyprogesterone 

Medroxyprogesterone is available in the U.S. through pharmaceutical manufacturers and through veterinary custom compounding companies. MEDROXYPROGESTERONE 200 MG/ML by NexGen Pharmaceuticals may be prescribed for behavioral control in horses and to stop mares from coming into season.

Medroxyprogesterone carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen.

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1Crowell-Davis, S. Sexual behavior of mares. Hormones and Behavior 52 (2007) 12–17.

2Beach, F.A., 1976. Sexual attractivity, proceptivity and receptivity in female mammals. Horm. Behav. 7, 105–138.

3Plumb’s Veterinary Drugs.

4Dascanio J. Hormonal Control of Reproduction. Proceedings: ABVP 2009. 2009. Veterinary Information Network.