Most people in the United States accept the notion that it takes two to tango in bed. But ever since the birth control pill received government approval 50 years ago, women have been waiting for the other shoe to drop -- when men get a pill, patch or injection to bear the burden of birth control.
A large study published Monday in the Journal of Clinical Endocrinology and Metabolism indicates that science may be bringing us closer to a male "pill," although the idea of hormones for male birth control still stokes a heated debate among doctors.
Researchers recruited more than 1,000 Chinese men who had fathered at least one child two years before the study with their partners to receive injections of testosterone undecanoate (TU) in tea seed oil every month.
After two years, the injections had 95 percent effectiveness for preventing pregnancy overall, and about 98 percent effectiveness for men whose sperm levels dropped off at expected levels within the first few months of the trial.
Male contraceptive researcher Dr. John K. Amory was impressed by the ability of the injections to drop sperm counts.
"The goal is to try to get to 99 percent effectiveness [for pregnancy prevention]," said Amory, who is an associate professor of medicine at the University of Washington in Seattle.
"They went down to 1 million sperm in a milliliter of semen from the normal 20 to 200 million sperm per milliliter," he said.
Although testosterone usually triggers sperm production, Amory said too much testosterone can shut down the testes.
"If you give external testosterone in the blood, this decreases the testosterone in the testes," Amory said. "It's because the body has what we call a feed-back loop. The brain sees the hormones and thinks, 'Oh, we're making plenty of that,' and shuts off [sperm] production."
The authors write that more long-term studies of testosterone injections are needed for safety, but "that these promising findings provide encouragement that male hormonal-contraceptive regimens may offer a novel and workable alternative to existing family planning options."
But not everyone agrees that hormones for male birth control are novel, or a workable alternative.
"It's really not new and it's been abandoned for a number of reasons," said Dr. Larry Ross, past president of the American Urological Association.
The first reason, Ross said, is that testosterone injections proved to be unreliable in the past.
"Failure rates as high as 20 percent have been reported," said Ross, who is also professor of urology at the University of Illinois at Chicago.
On the other hand, Ross said testosterone injections can be too effective and, over time, can sterilize a man. "There are not an insignificant number of men who will develop permanent sterility," he said.
Ross said that while men might not see any damage from low testosterone levels in the testes in the short term -- for 6 months or a year -- any longer and the testes can begin to scar.
"Places we've seen this is in steroid abuse like weight lifters, and there's been a lot of talk about baseball players lately," Ross said.
Indeed, Ross said, discussions about testosterone injections for male birth control at urology conferences largely leave out testosterone talks these days.