The female birth-control pill contains synthetic forms of the hormones estrogen and progesterone, which prevent the ovaries from releasing a monthly egg for fertilization during a regular process of ovulation (see How Menstruation Works to learn more). When male birth control research first got underway, pharmaceutical companies likewise focused on a hormone-based approach to curtailing sperm production.
Testosterone is responsible for the development of sexual characteristics, including muscle mass and facial hair growth, in men. It’s also responsible for stimulating the testes to manufacture sperm. The illustration demonstrates how the normal process works.
Normally, at the start of the sperm-production cycle, the hypothalamus in the brain releases gonadotropin-releasing hormone (GnRH), which triggers the release of fertility hormones called gonadotropins (FSH and ICSH) from the pituitary gland. ICSH signals the testes to produce testosterone, and FSH and testosterone tell the testes to produce sperm. When sperm production is complete, the testes release testosterone and a hormone called inhibin into the body, which tells the brain that there's enough sperm. This stops the release of sperm-producing hormones, until the testes require a refill, and the hypothalamus starts the cycle anew.
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].
These days, male birth control researchers have largely given up on finding a hormonal sperm blocker in favor of non-hormonal options.