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Estradiol Cypionate for Horses (foals)

By Barbara Forney, VMD

Last reviewed: 7/13/2022

Commonly prescribed for: Enhance Sexual Receptivity in Ovariectomized Mares

Species: Horses (foals)

Therapeutic Class: Endogenous Estrogen

Basic Information

Estradiol is the most active of the endogenous estrogens. Estradiol cypionate (ECP) is an esterified estrogen that is more potent and has a longer half-life than the predominant naturally occurring estrogen, estradiol 17 beta. The actions of estrogen hormones on the female include maturation of the reproductive tract and mammary glands, modulation of pituitary gonadotropin secretion, expression of estrus behavior, enhanced uterine-resistance to infection, gestational support during pregnancy and modulation of uterine contractions, and cervical relaxation during labor.


Commonly, ECP is most used in mares in order to enhance sexual receptivity in ovariectomized mares being used as stimulus mares in a semen collection program. ECP also may be used in a seasonally anestrous mare to produce sexual receptivity for the same purpose. Although the anestrous mare may be sexually receptive due to the effect of the ECP, if she is truly anestrous, she will of course not produce follicles or ovulate. ECP or other estrogens will not produce sexual receptivity in mares that are in diestrus with circulating progesterone.

Other less-common uses for ECP in the mare include:
  • Increasing sexual receptivity in mares that are in physiological estrus but still are not receptive to the stallion.
  • "Silent heats" can be a real management headache when natural service is necessary. The administration of exogenous estrogen has been tried under these circumstances, but research has not been done in the horse to prove whether or not there are any adverse effects on conception rates.
  • Some labor induction protocols include the use of ECP to soften the cervix prior to the administration of oxytocin.
  • Some veterinarians use exogenous estrogens on mares while treating uterine infections because research shows that the uterus is more effective at clearing infection while under the influence of estrogen.
  • Some researchers are working with exogenous estrogens or progesterone/estrogen combinations for pregnancy maintenance in mares that habitually suffer early embryonic loss.
  • Many of these treatments are still in the research stages and the most recent information should be consulted before embarking on treatment.

Estradiol Cypionate Side Effects

Injection site reactions may occur. No other side-effects have been reported when ECP is used in ovariectomized mares.


Estrogens generally are contraindicated in pregnant mares. As noted above there is some research on their uses in mares with a history of early pregnancy loss but our understanding of this process is still evolving.

Drug Interactions

  • Rifampin, phenobarbital, and phenylbutazone may decrease estrogen activity.
  • The effects of glucocorticoids may be enhanced when used with estrogens. The dose of corticosteroids may need to be reduced.


No information regarding overdose was found in the literature.
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