Male volunteers are being recruited for a new kind of contraception - an implanted capsule that can kill sperm production for up to three years.

This could provide a hassle-free method of birth control, but there is a considerable catch - users will have to give themselves an inter-muscular shot of testosterone once every four to six weeks to keep their sex drives normal.

The Dutch pharmaceutical company Organon revealed on Wednesday that they have begun the first clinical trials with male implants in medical centres throughout Europe and the US. Taking part are 120 men between the ages of 18 and 45. They will not have any trial results until the end of 2002, but they expect to have a marketable product by 2005.

Regarding the requirement for injections, Organon’s medical director Henk Out says: “It’s clearly not ideal. We’re definitely looking for alternatives.”

Match stick

The match-stick sized implant will be surgically embedded into the men’s arms, where they will release a sperm inhibiting progestogen hormone into the blood stream. This progestogen, called etonogestrel, is used in some female birth control pills and in trial versions of a male “pill”.

The progestogen is good at inhibiting sperm production. But it also suppresses the release of testosterone in the body. If testosterone levels drop below normal, that can kill libido and increase risks of osteoporosis. So users have to have a testosterone boost. Attempts to deliver testosterone through a pill or implant have so far failed, making a needle injection necessary.

The slow-release technology used to deliver the progestogen is the same as that used to deliver hormones in the female version of contraceptive implants. The capsules rely on diffusion of chemicals through different membranes to control the rate of release. The trial implants will be good for one year of contraception, but Organon plans to develop a three-year version.

Acne problem

Female contraceptive implants, such as Norplant or Organon’s Implanon, have not proven very popular in the past. Marianne Parry from the Marie Stopes family planning clinic in London says this is probably because the slow hormonal release tends to cause unpredictable menstrual bleeding. The implants can also cause headaches and acne and and require surgery to put them in or take them out.

No one yet knows if the male implant will suffer from similar difficulties. But Out says that a pill form of the same hormones has been shown to be effective and reversible with no ill effects.

Social acceptance is a further hurdle, says Parry. “I don’t know how willing men are going to be to bite the bullet,” she says. “But it’s good we can share the responsibility between the sexes.”