After more than a decade of work, government researchers in the U.S. are ready to test an unusual birth control method for men—a topical gel that could prevent the production of sperm.

And no, gentlemen, you don’t rub it on your genitals.

The clinical trial, which begins in April and will run for about four years, will be the largest effort in the U.S. to test a hormonal form of birth control for men.

Currently, the most effective options men have for birth control are condoms or a vasectomy. In the last major study of a hormonal male contraceptive, which took place in Europe from 2008 to 2012, participants received injections of hormones every two months. The shots suppressed sperm production and prevented the men’s female partners from getting pregnant, but they also gave men severe mood swings and other serious side effects (see “Why We Still Don’t Have Birth Control Drugs for Men”).

The new gel contains two synthetic hormones, testosterone and a form of progestin. Progestin blocks the testes from making enough testosterone to produce normal levels of sperm. The replacement testosterone is needed to counteract the hormone imbalances the progestin causes but won't make the body produce sperm.

More than 400 couples will participate in the study, which will take place at sites in the U.S., the U.K., Italy, Sweden, Chile, and Kenya. Men in the trial will take home a pump bottle of the gel and rub about half a teaspoon of it on their upper arms and shoulders every day. The gel dries within a minute.