Other studies have looked into using progestin, a synthetic hormone used in female contraceptives, Blithe said.
"You're shutting down the body's normal production of testosterone," she said. "When you do that, you shut it down in the testes, but you also shut it down in the blood. But if you don't replace the concentration in the blood, a lot of other things get turned off, such as libido, ability to achieve erection and building muscle mass."
Attempts to create an oral pill using testosterone to shut down sperm production have not worked because the body absorbs the male hormone before it can get the job done. "It gets metabolized pretty aggressively by the liver," Amory said.
However, studies have found success through injecting the hormones or applying them through the skin.
Chinese researchers, for instance, have shown that injectable testosterone can be effective in male birth control. Once a month for 30 months, the doctors injected 1,045 healthy, fertile Chinese men, 20 to 45 years old, with 500 milligrams of testosterone undecanoate mixed in tea seed oil.
The contraception proved effective, with a pregnancy rate of just 1.1 per 1,000 men in a two-year span. Additionally, the men reported no serious side effects and returned to normal fertility after they stopped taking the injections. The findings were published last June in the Journal of Clinical Endocrinology & Metabolism.
Amory is part of a research team investigating another means of hormonal birth control for men -- a gel that would be applied to the skin. The team is enrolling men for the study, and results could be more than a year away.
Though both are promising avenues of research, Amory warned that people shouldn't get their hopes up about a male "pill" anytime in the near future.
"Everybody's been saying, 'within the next five year
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