Though birth control pills have been available to women for nearly 60 years, there's nothing equivalent on the pharmacy shelves for men. But that may change soon: Yesterday (March 25), a team of scientists announced that its unique take on a male birth control pill passed human safety tests in a 28-day trial without any participants dropping out from side effects — a problem that has stymied other male birth control attempts.

So, how does the pill work?

The researchers attribute their successful trial to the active agent in the pill, which is two hormones in one. Part progestin and part modified testosterone, the hybrid molecule means that the consumer always has matching levels of the hormones in the body. [Future of Fertility Treatment: 7 Ways Baby-Making Could Change]

The coordination of these two hormones can help dodge low sex drive or other health problems that modified homeone levels can create, said Dr. Christina Wang, the associate director of the Clinical and Translational Science Institute at Los Angeles Biomed Research Institute (LA BioMed). Wang worked on the trial with researchers at the University of Washington School of Medicine in Seattle.

When the two hormones are separate, the body processes identical doses at different speeds, Wang told Live Science. Progestin stops sperm production, but it also decreases natural testosterone levels; and if testosterone drops too low, the odds of blood clots, depression and other problems rise.

"We want [the hormones] to come on and decrease roughly together," Wang said. Since this pill always pairs progestin with something similar to testosterone, she said, the molecule will ideally keep sperm count low while also making sure there's enough of the modified sex hormone to keep its essential roles filled.

During the 28-day study, participants took a pill with either 200 or 400 milligrams of the active ingredient, or they took a placebo. The trial aimed to evaluate only the safety of the drug, not whether it worked (it would take 60 to 90 days for sperm counts to go down).

None of the men showed some of the more serious side effects that could come from too-low testosterone levels, such as higher blood pressure or depression. But the participants weren’t completely side-effect free. Of the 30 study participants taking the pill, 22 reported acne, headaches, lower sex drive, mild erectile dysfunction or tiredness, and there was an average weight gain of 2.8 or 4.2 lbs., depending on the dose. (Three people who received the placebo pill also had some complaints.)

As for what health effects might appear when this pill is taken long term, that's still unclear, Wang said. Current studies in rats and monkeys are assessing whether taking the pill for three months or longer would have health effects, and once those studies are complete, a study of similar length will happen with humans, she said.

But ultimately, the goal is to transform the pill into an injection. "Not all men want to take a pill every day," Wang said, adding that a chemical very similar to the one in the pill has already been shown to linger in monkeys.

The findings were presented March 24 at ENDO 2019, the Endocrine Society's annual conference, and the study was published Feb. 1 in The Journal of Clinical Endocrinology and Metabolism.

Originally published on Live Science.