FRANKFURT, Germany (AP) -- Pharmaceutical companies Schering AG and Organon said Thursday they aim to produce a marketable hormone contraceptive for men in five to seven years -- a goal that has stumped researchers for decades.

Trials for the product -- a long-term implant accompanied by an injection every three months -- will start next year, with the goal of marketing the product in the United States and Europe, the companies said.

"A hormonal form of fertility control for men would expand the choice of contraceptives for couples," Werner-Karl Raff, head of Schering's contraceptives division, said in a written statement. "We are optimistic that we can fill this gap in the future."

The implant would contain a hormone belonging to the progestin class, which would stop production of both sperm and the male hormone testosterone. Injections of a testosterone derivative would replace natural levels of the hormone, necessary for normal sexual functioning.

Organon, which bases its top management in Westland, New Jersey, but has its biggest facilities in Ostend, the Netherlands, would make the implant - a version of its already existing Implanon female birth control implant.

Berlin-based Schering would produce the injected testosterone derivative, known as testosterone undecanoate. The companies could not say how much the product would cost, how much they expected to earn from it, or how it would be marketed.

Schering spokeswoman Astrid Forster acknowledged that some might find the delivery method inconvenient, requiring visits to the doctor to have the implant inserted in the upper arm and to have the injections.

In trials funded by Organon and done by University of Edinburgh researchers of 66 men in Scotland and China, the drug stopped sperm production but produced minor side effects such as mood swings, weight gain and increased appetite.

Researchers in Australia also say they have successfully tested a progestin-testosterone combination.

Schering and Organon said they would use the development period to try to come up with a more convenient way to administer the doses. "Of course, we know it's not very comfortable," she said. "But these are the first steps, and we will try to learn from this project and optimize it for the future."

The implant was intended to last a year during the trial phase, the companies said.

Researchers say that most major pharmaceutical companies have not shown much interest in developing and marketing male contraceptives. They also cite a biological obstacle - female birth control pills must stop the production of one egg a month, as opposed to millions of sperm produced every day.

But Schering's Forster said that the number of men who have had vasectomies suggests there would be demand for the product -- which would be easily reversible.

She said that the tests would provide more information, but that it was expected that fertility would return in three months after ceasing use.

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