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Progesterone in Oil for Veterinary Use

by Barbara Forney, VMD

Basic Information

Progesterone is a steroid hormone normally produced by the corpus luteum and by the placenta of pregnant mares. Exogenous progesterone is used to prevent mares from coming into heat, to synchronize estrus cycles of mares for better breeding efficiency, to organize or to regulate heat cycles during the mare's seasonal transition, and to help maintain pregnancy. It also is used to modify estrus-related behaviors that interfere with performance and pleasure riding in non-breeding mares.


When injectable progesterone is used in the non-pregnant mare to regulate the estrus cycle or synchronize estrus, it will suppress outward signs of estrus, although it will not prevent ovulation consistently. Consequently prostaglandin commonly is given at the end of a course of progesterone to cause lysis of the CL and predictable return to estrus. Injectable progesterone commonly is used for pregnancy maintenance and is accepted as safe in the pregnant and lactating mare. Pony breeds are similar to horses in their response to progesterone.

Progesterone in Oil Side Effects

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


  • Progesterone should not be used in mares with chronic uterine infections.
  • The long-term use of progesterone may delay the return to normal reproductive cycling in mares.

Drug Interactions

Rifampin may decrease progesterone's activity.


No specific information was found in the literature regarding overdose in the mare. Occasionally pregnant mares that have undergone surgery or other major stress are given very high doses in an attempt to protect their pregnancy. Doses of one to two grams of injectable progesterone in oil have been given in clinical situations with no apparent detrimental effect.

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Therapeutic Class
Steroid hormone


May Be Prescribed by Vets for:
Control of reproductive cycle, pregnancy maintenance and behavior modification.

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About the Author

Dr. Barbara Forney

Barbara Forney, VMD

Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.

She began to develop her interest in client education and medical writing in 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.

Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.