A new kind of male birth control may be on the horizon.

The largest clinical trial to date on hormonal male contraception is slated to begin in the first half of 2018, and aims to enroll more than 400 couples in six countries around the world. Men will rub a gel containing synthetic hormones into their upper arms and shoulders once a day, and researchers will track the gel’s effectiveness at preventing pregnancy in their partners.

The trial, which is sponsored by the National Institutes of Health and the non-profit Population Council, will include couples in the United States, the United Kingdom, Italy, Sweden, Chile and Kenya. The experimental gel contains a synthetic progestin called nestorone—which blocks the testes from making enough testosterone to produce sperm—and a synthetic testosterone, which will counteract subsequent hormonal imbalances.

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A different combination of progestin and testosterone was previously tested in a smaller clinical trial published last year, in which men got hormone injections every two months. Researchers found that the shots were 96% effective at suppressing sperm counts, but study enrollment was stopped earlier than expected because of complaints from men in the study about mood swings, muscle pain and other side effects.

Dr. Min Lee, program officer on the upcoming trial and a researcher at the National Institute of Child Health and Human Development, says that the contraceptive gel contains much less synthetic testosterone than the injectable formula did—which will hopefully prevent many of those problems. “If we stop the production of testosterone, men are obviously going to experience side effects beyond just their sperm count,” he says. “But we’re only adding back just enough so they have normal physiological responses, not the huge amount that’s been used in other formulations.”

A topical nestorone/testosterone combination was also shown to be effective in a 2012 six-month clinical trial, but that study involved two different gels that had to be applied to different parts of the body, says Lee. Over the last few years, he says, researchers at the NIH and the Population Council have reformulated the gels into one product.

The women in the trial will also use a form of female birth control for about the first four months their male partners are using the contraceptive gel. But once the men’s sperm counts drop below the threshold needed to prevent pregnancy—less than one million per milliliter—the couples will use only the gel as contraception for one full year.

The gel has been shown to suppress sperm levels for about 72 hours. That means “there is a bit of forgiveness” if men forget a dose, Regine Sitruk-Ware, distinguished scientist at the Population Council, recently told MIT Technology Review.

Even if the drug is successful, says Lee, it will likely be at least five years before it is approved by the Food and Drug Administration and becomes available to the public. Once the upcoming Phase IIb trial is complete, a much larger Phase III trial will need to be initiated.

But he is very hopeful about the future of male contraception, and says this new study is an important step toward making it a reality. His team’s research also shows that there is a demand for the product.

“If you had asked me two years ago if this was ever going to be a useful contraceptive, I would have hesitated and said, ‘Who wants to slather this gel all over their body?’” says Lee. “But in our interviews, men have told us they are more than willing to do this. There’s definitely a lot of enthusiasm, both from the investigators running the trials to the people participating in them.”

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