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The latest effort to come up with a male birth control pill has found a formulation that appears to be safe.

But the experimental pill has at least one of the same problems that plague female birth control pills: it caused the men to gain weight. And after just a few weeks of testing, it’s also not yet clear how well it works.

Even though the researchers said they were “very excited” by the results, they haven’t been testing it long enough to show whether it decreases sperm production, and they haven’t shown whether it stops couples from conceiving.

Nonetheless, it may be the best hope yet for a non-permanent male contraceptive, said Stephanie Page of the University of Washington School of Medicine.

“Our last great advance in male contraception was 300 years ago with the development of the condom,” Page told a news conference.

The pill Page’s team tested is called dimethandrolone undecanoate (DMAU) and it‘s tweaked version of previous failed efforts to develop a male pill. It is being developed with funding from the National Institutes of Health.

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The last big hope for a male birth control pill fell apart in 2016 when the drug was shown to damage the liver. It also required two doses a day.”It’s hard enough to remember to take a pill once a day,” Page said.

Her team tested 100 male volunteers and came up with good results from 82 of them. Some got sugar pills and the rest got varying doses of DMAU.

“Individuals on all the doses had remarkably low testosterone levels,” Page said. And the levels recovered within days of stopping the pills, the team told a meeting in Chicago of the Endocrine Society.

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More importantly, there were no major side-effects. “Despite testosterone levels that were very low, the men had no symptoms,” Page said. “No hot flashes — the men had normal mood,” she added.

“We did see some mild weight gain and a little bit of a change in cholesterol levels,” she added. The men gained between 3 and 9 pounds, and their HDL or “good” cholesterol levels fell slightly, something that might, over time, raise heart disease risk.

Plus the pills must be taken with food or they won’t work.

The next step is to see if the pills decrease sperm production. The team is starting that trial now that they’ve shown the drug is safe in the short term. Then they’ll have to test it in men who are married or in other long-term relationships with women.

“Our last great advance in male contraception was 300 years ago with the development of the condom."

DMAU works by interrupting the signal between the brain and the testicles, Page said. “Testosterone production is blocked and therefore sperm are not able to finish their last stages of maturation,” she said.

One big question has always been whether men would actually take a birth control pill reliably, since they’re not the ones who bear the burden of pregnancy.

“We’re often asked if there was a contraceptive if men would be interested in using it,” Page said.

“There has been very nice work in this area demonstrating that men across the globe — various races, ethnicities and across socioeconomic groups — are actually very interested in contraception,” she added. “And 60 to 80 percent of men surveyed in such studies say if there was a reversible contraceptive available, they would be very interested in using it.”