Since testosterone provides that signal that sperm production is complete, giving a man extra testosterone can serve the same purpose. But if there are always high doses of testosterone in the male bloodstream, they continually tell the brain that the testes are producing enough sperm, which turns off the release of GnRH indefinitely. Male birth control researchers discovered that this testosterone-pumping, GnRH-thwarting approach also comes with a host of physical side effects, including acne, weight gain, prostate-gland growth and abnormal liver function.
To solve the testosterone problem, researchers in the mid-2000s introduced progestogen, another synthetic sex hormone also found in female birth control, into the mix. The resulting male birth control method combined testosterone implants to inhibit sperm production and regular progestogen injections to counteract the unwanted side effects in 80 to 90 percent of male trial participants [source: Amory, Page and Bremner]. But since pharmaceutical companies doubted men would go to such lengths for birth control, they pulled the plug on funding [source: Goodman].