By James Dagwell,
BBC Three News Reporter
Trials of a male contraceptive have started in the UK - but scientists are having trouble recruiting men to take part.
Dr Bouloux is running the trial
The contraceptive combines an implant under the skin and regular injections to lower sperm production temporarily.
But too few men are coming forward, and experts are worried lack of interest could put a stop to a male pill before it's even introduced.
Experts say men simply need encouragement to try the contraceptive.
Dr Pierre-Marc Bouloux from the Royal Free Hospital in London began recruiting men for the trial in February.
It's placed under the skin of the upper arm. It slowly releases progestin into the bloodstream, which in turn switches off the production of hormones in the pituitary gland, responsible for stimulating sperm production.
He said the implant was "virtually free" of side effects, but no one knew what the consequences of long term use would be.
Mary Boyle, a clinical psychologist at the University of East London, says she is not surprised men don't want to take part. She thinks there would have to be a huge shift in male attitudes to contraception for it ever to be successful.
Ms Boyle says men simply don't believe they should be responsible for it.
She said: "There's a very strong idea that women should suffer more in the service of preventing pregnancy than men - that's a real serious barrier to any male contraceptive."
She also questioned whether women would be willing to trust men when they say they're on a contraceptive.
But the Family Planning Association says we should stop being so damning to men - it reckons men will want to take it once it's available.
Toni Belfield of the FPA said: "We haven't encouraged men to share responsibility -instead we should encourage men to feel part of sexual activity and the repercussions of sex. Men do want to take part, male contraception will allow them to do that."
A study of Scottish women by Edinburgh University suggested women would trust a man if he said he was taking contraception. Ninety per cent of women thought a male pill was a good idea, and 65% felt the responsibility for contraception falls too much on women.
When the female pill was introduced 40 years ago the long-term side-effects were unknown, but now women know all about the effects hormonal contraception can have on their bodies.
Mary Boyle said: "When the first health scare comes - as it will - what arguments are going to be used to persuade men to stay on it.
"Women can avoid pregnancy by being on the pill, but that argument doesn't wash with men."
Watch more on this story on the news on BBC Three, Friday 21 May at 19.00BST.