The 350-patient trial will be one of the largest ever investigating a hormone fertility control method for men. It will evaluate six dose combinations of the implant, based on the progestogen etonogestrel, which is delivered alongside injections of testosterone undecanoate.

The etonogestrel implant suppresses sperm production in men but, because it also reduces testosterone, injections of the long-acting hormone testosterone undecanoate are required to prevent side effects.

However, acknowledging the difficulties facing the programme, the companies said: "the present challenge is to develop a method that is not only reliable, safe and reversible but also acceptable to users." It remains to be seen whether there will be a demand for an injectable product for men.

Schering and Organon formed an alliance for this project in 2002, after running competing research programmes for some years. At the time, they said that a commercial product based on the technology could be on the market in as little as five years.