An attempt to develop a safe and effective "male pill" is making headway, according to preliminary results of a small study. In a four-week trial of men under 50, an experimental hormone-based birth control pill was found to be "well-tolerated."

And participants' testosterone levels dropped significantly along with two hormones essential for sperm production, the U.S. study team noted.

Study author Dr. Stephanie Page described the results as "a promising step forward" in the development of a male version of the female birth control pill.

But don't ditch your condoms just yet. Page added that "larger, longer-term studies are clearly needed to address potential side effects."

Interest in a male birth control pill is strong, she said.

"Women have many options, but many women cannot use hormonal and other methods available to them," said Page, who heads the University of Washington's division of metabolism and endocrinology, in Seattle.

"Men are increasingly interested in sharing the burden of contraception, as well as controlling their own fertility," she noted.

For the study, researchers evaluated three doses (100, 200 and 400 milligrams) of a once-daily contraceptive prototype called dimethandrolone undecanoate (DMAU). Two formulations were tried inside the capsules, either powder or castor oil.

DMAU combines activity of a hormone like testosterone and a progestin. It was developed by the U.S. National Institutes of Health, which funded the study.

It differs from prior attempts to create a male birth control pill in several ways. It contains just one steroid, rather than two, and was not associated with any liver toxicity, something that has plagued prior male pill efforts, Page said. And "unlike other oral testosterone derivatives, DMAU only needs to be dosed once a day," she added.

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