Last year, researchers at the National Institutes of Health launched a Phase IIb clinical trial to investigate the male contraceptive gel NES/T, which combines segesterone acetate (Nestorone), a progestin hormone used for female birth control, with the male hormone testosterone.

NES/T is a reversible contraceptive designed to be absorbed through the skin on men’s upper arms and shoulders. The female hormone in the gel blocks the production of testosterone and lowers or stops sperm production in the testes while the male hormone in the product supports a man’s normal sex drive and other testosterone-dependent functions.

Scientists enrolled about 420 couples into the study. Male volunteers were given the gel daily for four to 12 weeks to see how well they tolerated NES/T and whether they experienced any unacceptable side effects. If sperm levels didn’t decline, researchers requested that men continue to apply the gel for up to 16 weeks to achieve this goal.

Next, participants are slated to enter the trial’s efficacy phase, scheduled to last 52 weeks. During this time, couples will be asked to use the gel as their only birth control method so that scientists can determine NES/T’s ability to prevent pregnancy. (Men will remain in the study for an additional six months for researchers to check for any long-term effects.)

The study is expected to be completed in September 2021.

“Expanding male contraceptive options could help make family planning more of a shared responsibility between women and men,” says Régine Sitruk-Ware, MD, the codirector of the trial.