The U.S. Food and Drug Administration warns that exposure when treating a horse with synthetic progesterone may cause serious health risks. The drug may cause adverse events, including reproductive system disorders, which have occurred in teenage girls.
Altrenogest suppresses estrus in mares. Merck and other companies market the horse progestins as:
- Regu-Mate or Regumate,
- and Altren.
The equine products are available with a veterinarian’s prescription. They are administered orally by applying directly on the base of the mare’s tongue or on the mare’s feed.
The FDA reports it received 130 reports of accidental human exposure to altrenogest products from October 6, 1987, to May 30, 2018. Additionally, the majority of complaints  concerned Regu-Mate.
Furthermore, 137 people reported adverse effects, including 18 teenage girls. Some were as young as 14-years-old. Exposure often involved the drug making contact with the skin. Although, some reported product residue exposure from barn surfaces, equipment, and treated animals.
Reproductive adverse effects reported in females include abnormal or absent menstrual cycles, and in men include decreased libido. Other adverse effects reported include headaches, fever, abdominal pain, nausea, diarrhea, vomiting, and rashes.
Intact skin absorbs Regu-mate. Pregnant and women of childbearing age should exercise extreme caution, according to the FDA, if they can’t avoid handling the drug. Anyone working with Regumate should wear non-porous gloves.
The FDA and Merck are discussing package labeling modifications to enhance product safety. Merck owns the rights to Regu-mate.
If approved, the changes would have to be made to the other products as well due to the law.