Hopes for a male contraceptive pill have increased after a trial showed a once-daily capsule appeared to work with no significant side-effects.

The drug aims to suppress levels of hormones that drive the production of sperm and testosterone in the testes.

In the month-long study, doctors found that levels of the hormones required for sperm production fell in men who took the daily capsule, suggesting their sperm counts had been markedly reduced. The next step will be to confirm the fall and determine if it is sufficient.

Scientists have been searching for a male contraceptive pill for years, testing numerous pills, injections and gels to find an alternative to condoms and vasectomies.

Stephanie Page, a professor of medicine and co-senior investigator on the trial at the University of Washington in Seattle, said: “The goal is to expand contraceptive options and create a menu of choices for men like we have for women. We are neglecting a major potential user population with the limited options currently available to men.”

Despite the range of contraceptive pills, patches and injections available for women, and the choice of either condoms or vasectomies for men, the rate of unplanned pregnancies had stagnated at about 40%, Page said.

Forty healthy men enrolled in the phase one safety trial at the University of Washington and LA BioMed. Ten volunteers received a placebo capsule; the rest were assigned a 200mg or 400mg dose of the drug, 11-beta-MNTDC. All took one capsule daily with food for 28 days, according to a presentation at the Endocrine Society’s annual conference in New Orleans.

The drug contains a form of progesterone that blocks the production of hormones called LH and FSH that are needed to make sperm. It also has a testosterone-like compound, or androgen, which balances a drop in the male hormone caused by the progestin.

Page said: “Since testosterone production is shut down in the testes, the androgen action in the rest of the body maintains ‘maleness’ elsewhere, supporting things like male pattern hair, deep voice, sex drive and function, and lean body mass.”

Men who took the drug had substantially lower levels of LH and FSH compared with those who had the placebo, suggesting the drug had stopped their sperm production.

Christina Wang, a co-senior investigator on the trial at LA BioMed, said the next step was to measure the fall in sperm production directly. “This study is very short and we need three months if not more to stop sperm production,” she said. “All we have shown so far is that it shuts down the hormones that control the function of the testes.”

While none of the men dropped out or had less sex during the trial, some experienced mild side-effects. Four to six men reported either headaches, fatigue or acne, five experienced a mildly decreased libido, and two had mild erectile dysfunction. Similar side-effects have hampered other experimental male contraceptives. In the latest trial, some men also put on weight, Wang said.

The researchers say it could be a decade before a male pill is on the market, but they believe there is strong demand from couples. “Men have really limited options when it comes to reversible contraception,” Wang said. “When we ask men about hormonal compounds, about 50% are willing to try this new method. And when you ask their partners, the percentage is even higher.”