How Will the Male Pill Work?

The ideal male birth-control pill would eliminate or block the majority of sperm a man produces without causing significant side effects or affecting a man's masculinity or sexuality.

Testosterone molecule

One of the main avenues for male birth control is the hormonally-based pill. And one of the main hormonal ingredients under investigation is testosterone, which is responsible for the development of sexual characteristics, including muscle mass and facial hair growth, in men. It is also responsible for sperm production in the testes. This is how the normal process works:

Giving a man extra testosterone can essentially shut down sperm production. 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. Then, the cycle begins again.

But if there are always high doses of testosterone in the bloodstream, they continually tell the brain that the testes are producing enough sperm, which shuts off the release of GnRH indefinitely. The trouble with this method is that giving a man the amount of testosterone necessary to suppress sperm production can cause several unwanted side effects, including acne, weight gain, prostate-gland growth and abnormal liver function.

Drawback One problem with a hormonal approach to contraception in men is that it would take several months for the birth-control protection to take full effect.

A more promising alternative uses a combination of testosterone and another sex hormone: progestogen (progesterone). In low doses, progestogen suppresses the reproductive hormone system of both men and women -- progestogen is a component of the female birth-control pill. In men, it inhibits sperm production in the testes. The drawback is that it can also affect male sexuality and sexual characteristics, so men who take progestogen must also have testosterone injections to maintain those characteristics.

Theoretically, the male "pill" would contain both progestogen and testosterone. But because digestion breaks down testosterone, a combination pill couldn't hold enough testosterone to be effective at reversing the effects of the progestogen. So researchers are conducting studies with a progestogen implant combined with testosterone injections (see Beyond the Pill).