Scientists may have found the recipe for a male pill.
In tests, the drug protected against pregnancy without affecting sex drive or producing other unwelcome side-effects.
The US research paves the way for a male version of the contraceptive pill taken by millions of women around the world.
Taken daily or weekly, it would allow couples to share the burden of family planning.
A healthy man makes 1,000 sperm with every heartbeat – and just one is needed for conception
Importantly, the results of early experiments suggest the effects would be temporary, with fertility quickly rebounding on stopping taking the pill.
The scientists said: ‘The lack of contraceptive alternatives for men is partially responsible for the high rate of unplanned pregnancies, especially in teenagers, and contributing to the maternal mortality, ethical, social and financial costs associated with abortions and deliveries to single mothers.’
Others pointed out that if the researchers are successful, they will have created the first effective, non-permanent, male contraceptive since the advent of the condom, centuries ago.
However, it remains to be seen whether women would trust their partners to take the drug.
And whether men would want to take a medicine that reduces their virility, even temporarily.
The quest for a male pill has been thwarted by the biology of the male body. Whereas a woman releases one egg per month, a healthy man makes 1,000 sperm with every heartbeat – and just one is needed for conception.
The search has also been dogged by side-effects such as hot flushes, mood swings and low sex drive, caused by hormones in the drugs disrupting the body’s own hormonal balance.
The new drug, which is known only as JQ1, disrupts a crucial stage of sperm development. It is hormone-free, and in tests on mice, it did not produce any side-effects.
Sperm production plummeted and what sperm were made were bad swimmers, the journal Cell reports.
The animals were still interested in sex but, when the dose of the drug was right, were unable to sire any pups.
However, when they were taken off the drug, their fertility quickly returned and they became father to normal-sized, healthy, litters.
Also in JQ1’s favour is the fact it could be given in pill-form. Most male contraceptives in development are jabs or patches.
James Bradner, of the Dana-Farber Cancer Centre in Boston, said: ‘Our findings demonstrate that when given to rodents, this compound produces a rapid and reversible decrease in sperm count and mobility with profound effects on fertility.’
Dr Bradner, who collaborated with scientists at Baylor College of Medicine in Houston, is now improving the recipe of JQ1, which was first designed to tackle cancer.
He has been given money from the US government to do this but cautions the need for extensive testing means any male pill is still a long way from the market.
Mike Wyllie, one of the team of scientists that discovered and developed Viagra, pointed out that in the tests on mice, the drug didn’t completely stop sperm production.
He said: ‘Market research has shown that many women would prefer to take responsibility for contraceptive control rather than rely on their male partners.
‘The issue for the relationship will be the reliability; both in the man taking the drug and complete suppression of sperm production.
‘Conception is an interaction between the egg and one sperm; the data show considerably less than 100 per cent effective contraception.’
Dr Allan Pacey, a fertility expert at Sheffield University, said that the ‘door is wide open’ for the development of a non-hormonal male pill.
He added: ‘Although the study has only been performed on mice, it should be fairly easy to test out this approach out on humans and see whether it works equally well.
‘This is impossible to predict in advance but it is certainly worth pursuing.’