Both men and women are responsible for pregnancy, yet the burden of preventing it often falls on one gender. Women can choose from a variety of options to control fertility while for generations, men have been limited to withdrawal, condoms and sterilization. But someday soon, a new method may allow men to shoulder a greater share of responsibility.
A new hormonal birth control shot for men effectively prevented pregnancy in female partners, a new study found.
The study, co-sponsored by the United Nations and published Thursday in the Journal of Clinical Endocrinology and Metabolism, tested the safety and effectiveness of a contraceptive shot in 320 healthy men in monogamous relationships with female partners. Conducted at health centers around the world, enrollment began on a rolling basis in September 2008. The men, who ranged in age from 18 to 45, underwent testing to ensure that they had a normal sperm count at the start.
The injection, given every eight weeks, consisted of 1,000 milligrams of a synthetic form of testosterone and 200 milligrams of norethisterone enanthate, essentially a derivative of the female hormones progesterone and estrogen referred to as "progestin" in the synthetic form.
According to Dr. Seth Cohen, a urologist at NYU Langone Medical Center, when a man is given a shot of testosterone, "basically, the brain assumes the body is getting enough," so the body shuts down its own production of testosterone -- specifically "the testicle's production of testosterone as well as the testicle's production of sperm."
The progestin "further drives the brain malfunction, so it stops the testicle's production of both testosterone and sperm," explained Cohen, who was not involved in the new study.
The researchers used a combination of hormones in order to reduce the testosterone dose to a level that they believed, based on previous studies, would effectively lower fertility yet still be safe.
During the ramp-up pre-efficacy stage of the study, the couples were instructed to use non-hormonal birth control methods, while the men participants received shots and provided semen samples until their sperm counts dropped to less than 1 million per milliliter in two consecutive tests. At that point, couples relied on the injections as contraception.
Throughout the study, the men provided semen samples to ensure that their sperm counts stayed low. Once the participants stopped receiving the injections, they were monitored to see whether and how quickly their sperm counts recovered to levels described as "fertile" by the World Health Organization.
The researchers discovered that the shot effectively held the sperm count at 1 million per milliliter or less within 24 weeks for 274 of the participants. The contraceptive method was effective in nearly 96% of continuing users.
Four pregnancies (resulting in three live births) occurred among the men's partners, all during the phase where other contraception was required. All the babies were normal, noted Doug Colvard, co-author of the study and deputy director for programs at the nonprofit research organization CONRAD, Eastern Virginia Medical School, a co-sponsor of the study.
