The findings presented at the Endocrine Society's annual meeting in Chicago showed that the new pill - called dimethandrolone undecanoate, or DMAU - appears to be safe when used daily for a month.
Like the pill for women, DMAU combines activity of an androgen (male hormone) like testosterone, and a progestin, and is taken once a day, said the study's senior investigator, Stephanie Page, Professor of Medicine at the University of Washington in Seattle.
"Many men say they would prefer a daily pill as a reversible contraceptive, rather than long-acting injections or topical gels, which are also in development," Page pointed out.
Progress toward a male birth control pill has been stymied because, according to Page, available oral forms of testosterone may cause liver inflammation, and they clear the body too quickly for once-daily dosing, thus requiring two doses a day.
However, DMAU contains undecanoate, a long-chain fatty acid, which Page said slows this clearance.
The study included 100 healthy men between ages 18 and 50 years.
The investigators tested three different doses of DMAU -- 100, 200, and 400 milligrams, or mg.
A total of 83 men completed the study, including giving blood samples for hormone and cholesterol testing on the first and last day of the study.
The low levels, Page said, were consistent with effective male contraception shown in longer-term studies.
"Despite having low levels of circulating testosterone, very few subjects reported symptoms consistent with testosterone deficiency or excess," Page said.
All groups taking DMAU did have weight gain and decreases in HDL (high-density lipoprotein, or "good") cholesterol, both of which Page said were mild.
All participants passed safety tests, including markers of liver and kidney function, the study said.
"These promising results are unprecedented in the development of a prototype male pill," Page said.
"Longer term studies are currently underway to confirm that DMAU taken every day blocks sperm production," she added.
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