USA — For decades, if not centuries, women have wished that men could be more involved in the process of birth control. Aside from the condom, no reliable method of contraception has been available for use by men, leaving women primarily, if not entirely, responsible for protecting against unintended pregnancies. Attempts to control male fertility via pills or injections have met with limited success — but now scientists believe that a safe and effective method of male birth control may be on the horizon.Because millions of sperm must be controlled, as opposed to a single egg, developing an effective chemical contraceptive that doesn’t interfere with testosterone levels has been elusive. Recent trial tests on laboratory animals have indicated that a reversible contraceptive for men without long-term side-effects may well be available in the near future, with human trials beginning within the next few years.

The current wave of experimental male contraceptives focus on changing the level of sex hormones in a man’s bloodstream, thus decreasing fertility. Among the promising pills is one made by combining progesterone, a female hormone know to inhibit the creation of sperm, and testosterone, which resists the undesirable effects of progesterone.

Another approach is being developed by Chuen Yan Cheng of New York’s Population Council. Unlike previous developers, Cheng has been working on an implant instead of a pill. The implant in question uses the chemical adherin, which breaks down the connection between Sertoli cells, which nurture immature sperm cells in the testes.

“It only works at the site of attachment with the Sertoli cells,” Dr. Cheng explains, “so they don’t affect the hormones of the male. This is a new and different approach to the hormone-based contraceptives.”

According to the results of a trail using laboratory rats that was published in the journal Nature Medicine, Cheng’s medication caused sperm levels to drop low enough to cause infertility in human males.

Unfortunately, adherin can also cause serious side effects, a problem Cheng and his associates addressed by binding it to a synthetic version of a male hormone known for its affection for Sertoli cells. In addition to removing the toxic element from adherin, Cheng’s contraceptive quickly exits the body upon removal or completion of the implant, allowing fertility to return to normal.