Plaster of Hemlock
Hemlock has had multiple uses in history. In the 18th century, it was thought to cure cancer, relieve syphilis, treat bacterial infections such as Mycobacterium tuberculosis and cure tumors, among other health benefits. The ancient Greeks used it for prisoner executions.
In the 13th century, Portuguese physician Pedro Julião -- who went on to become Pope John XXI-- wrote a book, "Thesaurus Pauperum" (which translates to "Treasure of the Poor"), which was a collection of herbal remedies for the poor who needed medical knowledge or attention but didn't have the funds to visit a medical professional. Included in his herbal remedies were prescriptions for birth control methods, for both men and women, including one for a male contraceptive: a poultice or plaster of hemlock applied to the testicles before sex, which would allegedly make the testicles shrivel and diminish the libido.