Researchers have finally found a compound that may offer the first effective and hormone-free birth control pill for men. The study in the August 17th Cell, a Cell Press publication, shows that the small molecule makes male mice reversibly infertile without putting a damper on their sex drive. When the animals stop taking this new form of birth control, their sperm rebound and they are again able to sire perfectly healthy offspring.

“This compound produces a rapid and reversible decrease in sperm count and motility with profound effects on fertility,” said James Bradner of the Dana-Farber Cancer Institute, the lead author of the study.

A male birth control pill hasn’t been easy to come by in large part because of the challenge of getting any drug across the blood:testis barrier, where it can reach the sperm-generating cells. That lack of contraceptive alternatives for men is partially responsible for the high rate of unplanned pregnancies. Despite the unsatisfactory options for male contraception, nearly one-third of couples rely on male-directed birth control methods.

Known as JQ1, the compound developed in the Bradner lab works by targeting a testis-specific protein called BRDT that is essential for fertility. When mice are given the BRDT-inhibiting molecule, they begin producing fewer sperm and those sperm they do produce don’t swim as well.

“This is a good reason to get excited about low sperm counts,” said Martin Matzuk of Baylor College of Medicine, another author of the report.

Mating studies confirm that JQ1 indeed works as effective contraception. Even better, those effects are complete and reversible and without adverse consequences for the animals’ testosterone levels or behavior. The small molecule also comes without any apparent adverse effects on the males’ future offspring.a