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Progesterone and Estradiol for Horses

By Barbara Forney, VMD

Last reviewed: 7/13/2022

Commonly prescribed for: Regulation of estrus, synchronization of estrus, delay of post foaling estrus, pregnancy maintenance

Species: Horses

Therapeutic Class: Steroid Hormone Combination

Basic Information

Progesterone and estradiol 17-beta combination (P and E) is commonly used as a management tool in broodmare practice. Demand for a reliable means of predicting or regulating estrus and ovulation in the mare has increased with the common use of cooled or frozen semen, embryo transfer, or other forms of appointment breeding. This combination is more effective in suppressing follicular activity during the treatment period than progesterone alone. At present this drug combination, when used with prostaglandin, is the most reliable means of estrus synchronization.


The most commonly followed protocol for estrus regulation or synchronization is once a day IM injection of 150mg of progesterone combined with 10 mg of estradiol for 10 days. On the tenth day prostaglandin is given in addition to the last P and E shot. Different studies vary in their results, but about 80% of mares given this treatment regimen will ovulate between eight and 10 days after the last injection. P and E also is used for pregnancy maintenance and is accepted as safe in the pregnant and lactating mare. P and E may be used to delay the first postpartum estrus. Pony breeds are similar to horses in their response to P and E.

Progesterone and Estradiol Combination Side Effects

The most-common side effect is injection-site reaction. These reactions usually respond to hot compresses and NSAIDs.


Compounds containing progesterone should not be used in mares with chronic uterine infections.

Drug Interactions

Rifampin may decrease progesterone's activity. It is difficult to envision an occasion when a broodmare would be on both rifampin and P and E.


No specific information was found regarding overdose in the mare.
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