The birth control pill for men is approaching the human phase of testing after 99% effective results were reported in mouse trials.

The American scientists behind the breakthrough belong to the University of Minnesota and presented their findings at the American Chemical Society’s 2022 Spring Conference in San Diego, California, Tuesday.

The secret to the pill, they said, is that testosterone, the male sex hormone, is not digested, reducing the risk of side effects that, until now, have kept drugs other than the condom out of pharmacies as contraceptive methods for men.

“Most birth control pills for women work with female sex hormones,” said Abdullah al Noman, one of the researchers, noting the lack of approved male oral contraceptives on the market.

“But targeting male sex hormones leads to a lot of side effects, such as weight gain, depression and increased risk of cardiovascular disease,” he said. “Men are less willing to take birth control pills, which have significant side effects. That’s why we’re aiming for a non-hormonal way to develop birth control pills for men.”

It should be noted that female hormonal contraceptives also cause many side effects, but this is not a barrier to their free sale by pharmaceutical companies, and their medical use is often recommended as a family planning method.

Although progress has been made in the development of non-hormonal oral contraceptives for women, there are currently no such products on the market or conclusive studies to support them.

For men, this nonhormonal contraceptive acts as an inhibitor of a protein called retinoic acid receptor alpha (RAR-α), one of a family of three nuclear receptors that bind to retinoic acid, a form of vitamin A that plays an important role in sperm formation, among other things.

The researchers found that elimination of the RAR-α gene in male mice rendered them infertile.

Although another study developed an oral compound that inhibited all three nuclear receptors, the University of Minnesota team wanted a drug specific for RAR-α to minimize side effects.

The researchers, led by Dr. Gunda Georg, found that a compound called YCT529 could inhibit only RAR-α. The researchers then administered YCT529 orally to male mice for four weeks and found that it dramatically reduced sperm counts and prevented pregnancy by 99 percent without causing any observable side effects.