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Progesterone in Oil for Horses

By Barbara Forney, VMD

Last reviewed: 7/13/2022

Commonly prescribed for: Control of reproductive cycle, pregnancy maintenance and behavior modification

Species: Horses

Therapeutic Class: Steroid Hormone


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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Progesterone: Capsule

Progesterone: Capsule

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