By Jeremy Laurance and Jillian Green
Birth control could soon be as much a man's responsibility as it is a woman's.
This after the first trial of a male contraceptive in sexually active couples has shown it was 100 percent effective in preventing pregnancies.
The hormonal treatment - a combination of an implant under the skin and a three monthly injection - was used by 55 couples for one year.
This method relieved the men of the need to remember to take a daily pill - and none of their partners became pregnant.
The research, by a team from Australia, is published in the Journal Of Clinical Endocrinology And Metabolism.
Professor David Baird of the Centre for Reproductive Biology at Edinburgh University, an expert on the male pill, said the finding was a significant advance.
"We do seem to be making progress to getting a male pill or some form of male contraception. There will be slight alterations to the components used, but it helps us on our way," he said.
Baird said there were six groups around the world testing versions of a male contraceptive focused on a combination of the male hormone, testosterone and the female hormone, progestogen.
Earlier trials on volunteers in the United Kingdom had shown that the hormonal combination prevented production of sperm in men, but the Australian researchers from the Anzac Research Institute in Sydney had been the first to show that it worked in a real situation when used by couples.
Professor David Handelsman, who led the research, said: "This shows the way for a final product to be a single injection containing testosterone and progestogen which will easily be given by local doctors on a three- to four-monthly basis and still maintain sexual health."
He said it was now up to the pharmaceutical companies to develop the research into a usable product.
Two companies, Organon and Schering, announced 18 months ago that they were working to develop a male pill. This is likely to be available either as an implant or in oral form.
Surveys have shown women are enthusiastic about the prospect of handing over responsibility for contraception to men - especially if protection is guaranteed.
Dr Nevon Ramsunder, managing director of Mens' Clinic International in South Africa, said: "I think it is a great achievement in modern medicine and it is about time that birth control be a shared responsibility in a relationship."
But Ramsunder added that one should consider what the implications would have on the spread of sexually transmitted diseases.
"A lot of men would start using this as a form of contraception in casual relationships and would stop using the conventional contraceptive like the condom which would have a direct effect on STDs and HIV/Aids," he said.
He added that a lot of research and trials will still have to take place, "so it is a bit premature to get too excited at this stage".