LONDON: Injections of the hormone testosterone could be an effective male contraceptive, according to a report issued Tuesday by the World Health Organization. Nine countries participated in a study of 399 men, aged 21 to 45, which showed that weekly injections of the hormone effectively prevented conception by decreasing sperm counts. Administered over two and a half years, the injections proved to be effective means of contraception for 98.6 percent of the couples involved, with only four pregnancies reported. "It would show that testosterone, like estrogen in a female, can be an additional method of reproduction control," said Richard Sherins, the director of andrology at the Genetics and IVF Institute in Fairfax, Virgina. "Testosterone may be a new way of looking at birth control," says TIME's Alice Park. "If it is true, it would be an exciting flipside as contraception is usually focused on the woman." But, she adds, two and one half years is not enough time to understand how hormone injections may effect a man's systems in the long term, or if they may lead to more prostate and testicular cancers. Researchers say it will be several years before the hormone contraceptive would become widely available.