A pill containing synthetic testosterone and progestin taken by men in a clinical trial conducted in Italy showed evidence of lowering males' sperm counts to very low levels (below 3 million sperm per milliliter of ejaculate). Half of the participants had sperm counts of zero after taking this pill. It was also demonstrated to cause few undesirable side effects. In another type of research study, testosterone injections were administered in conjunction with combined progestin and testosterone pills. This was shown to be more effective than use of testosterone injections alone.
Either way, this extra surge of testosterone is designed to interfere with a complex sequence of hormonal reactions that begin in the brain and end in the testes, resulting in sperm production. To get more technical, this extra dose of testosterone withholds the amount of gonadotropin-releasing hormone (GnRH) secreted by the hypothalamus in the brain. GnRH stimulates the release of hormones called gonadotropins from the pituitary gland, which is also in the brain. Gonadotropins are hormones essential in signaling the testes to produce sperm. So, the suppression of GnRH ultimately results in preventing sperm production in the male and, as a result, helps to prevent pregnancy in the woman partner.
Maximum contraceptive effectiveness, however, was not possible right away for any of the above mentioned combinations because it can take as much as 2 to 3 months for sperm counts to reach very low levels. This translates into a need for men or couples to plan in advance. In addition, the studies that involved a weekly injection schedule were shown to be particularly impractical. Because of this, other ways to improve the use and effectiveness of new male contraceptives are being investigated, including longer-acting male contraception that can deliver testosterone for 2 to 3 months. Also in development are testosterone-derivative implants and a vaccine that would be designed to prevent pregnancy for up to a year.