A gel containing a new hormone combination could make male birth control as easy as applying aftershave. In preliminary testing, the gel—which contains testosterone and a new version of progestin—was effective in reducing sperm count in up to 89 percent of men to very low levels.

This is “a level that is compatible with very low pregnancy rates," said study author Christine Wang, MD, from the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center.

Progestin is a synthetic version of a naturally occurring hormone, progesterone. In this study, researchers used a new form of progestin—Nestorone—with fewer side effects such as acne and changes in cholesterol levels. When combined, progestin and testosterone turn off the creation of hormones responsible for sperm production. Previous trials of these hormones have involved pills, implants, or shots.

In this study, men were asked to apply one of three gels to their skin each day for six months. Eighty-eight to 89 percent of men using a combination of testosterone and Nestorone developed a very low sperm count, compared to only 23 percent of men using a testosterone-only gel.

Some men—69 to 78 percent—using the new combination gel showed no sperm at all. Again, this was the case in only 23 percent of men using just testosterone. One advantage of a male birth control gel is that it would be reversible—sperm production would start again when men stopped applying the gel. If proven effective by larger trials, the new gel could also be easily applied at home.

Getting men to put on the gel each day, though, could be another challenge. In this study, only 56 percent of men stuck with the program for at least 20 weeks.