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.

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.”

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