The Off-label Use of Altrenogest in Stallions: A Survey

Within the equine industry, overly aggressive stallion behavior related to various factors including genetics, dominance, or mating can create a dangerous environment and present several training challenges, in attempt to reduce this behavior, some performance trainers are administering altrenogest, an oral synthetic progestogen (Regu-Mate®, Intervet Inc, Millsboro, Del) in an off-label use. Regu-Mate® is commonly used in the equine industry to suppress estrus in performance mares and to synchronize estrous cycles for breeding. Although several articles have been published concerning the possible negative effects of this off-label use of altrenogest in stallions on spermatogenesis and steroid production with varying behavioral effects, the use within the industry continues. However, there are currently no known estimates of the extent to which this exogenous hormone is being utilized in stallions. Our objective was to survey the rate of Regu-Mate use in Quarter horse stallions during 2002. Two hundred questionnaires were sent to Quarter horse stallion owners, trainers, and exhibitors in the United States. The answers presented give a type reference of the equine facilities that responded, as well as details concerning those that administered Regu-Mate®. Sixty-three of the surveys were competed and returned (32%), with Regu-Mate® use occurring in stallions identified by 18 of 63 facilities (29%). Most of the stallions receiving Regu-Mate® were not used for breeding at the time of dosing; however most were intended for breeding at a later point. The primary use of altrenogest given to stallions 2 years and older in a performance setting was to create a more focused animal.