The National Institutes of Health is looking for a few good men —and a few brave women — to try out a new birth control gel for males.

The gel, rubbed into the shoulders daily, gradually brings down sperm counts so that men cannot make a woman pregnant. The National Institute of Child Health and Human Development (NICHD), part of the NIH, is helping enroll about 400 couples at sites around the world to test how well the gel works to prevent pregnancy, and also check out how well people like it and whether men will use it as directed.

“This is the first time that men are using it as part of a couple to test for effectiveness,” said Diana Blithe, chief of NICHD’s Contraceptive Development Program.

The gel formulation, called NES/T, includes a progestin-containing compound called segesterone acetate, which is made under the brand name Nestorone, along with a dose of testosterone.

“It is applied to the back and shoulders and absorbed through the skin. The progestin blocks natural testosterone production in the testes, reducing sperm production to low or nonexistent levels,” the NICHD said in a statement.

It’s formulated as a gel because Nestorone does not get absorbed by the body when it’s taken orally, and testosterone does not stay in the body for a full day when taken as a pill. Both hormones last longer and work better when dosed through the skin.

There is no commercial male hormonal contraceptive on the U.S. market now. All men have to choose from are condoms or vasectomy.

"Worldwide, 85 million pregnancies (40 percent of all pregnancies) per year are unplanned, contributing to a higher incidence of adverse health outcomes for women and infants," the Population Council, which developed the product and which is helping test it, said in a statement.

Nestorone, combined with the hormone estradiol, is also used as a female contraceptive. A product was approved by the U.S. Food and Drug Administration in August as part of a vaginal ring for women to use as birth control.

How can the same hormone work as a contraceptive in both men and women? It’s because many common hormones have similar chemical structures, Blithe said.

In men, the hormone tricks the body into thinking it can stop making sperm. In women, it mimics pregnancy and tricks the body into thinking it can stop making eggs. “The male has very low levels of progesterone normally. Now they are exposed to a high amount, and that tells the testes, ‘oh there is a lot of steroid around so I don’t need to make more now’,” Blithe said.

Giving back some testosterone along with the Nestorone stops undesirable side-effects such as low libido and muscle loss, Blithe said.

“The potential of this new gel is huge,” said Dr. William Bremner of the University of Washington School of Medicine, who is helping test the gel. “There is a misperception that men are not interested in, or are even afraid of, tools to control their own fertility. We know that’s not the case.”

One question is how effective the treatment will be. Different birth control methods have different efficacies.

“If we are talking about a daily pill in women, if they use it perfectly, the failure rate is extremely low,” Blithe said. “In typical use the failure rate is 7 percent. With condoms, if they are used perfectly, the failure rate is low but in typical use the failure rate is 12 percent.”

In theory, men could forget to use the gel for a day with no consequences. “If they stop using it for three, four, or five days, then it won’t work the way it is supposed to,” Blithe said.

In the trial, men will use the gel until their sperm count falls low enough, and then their wives or partners will stop using their own birth control.

“Do I think they’ll use it? Certainly, they are capable of using it and they are willing to use it," Blithe said. "But people are human and people forget.”