Researchers say a male birth control is possible combining two hormones, one of which is used in female birth control.

A contraceptive containing the hormones testosterone and progestin has been shown to safely and effectively suppress sperm production in most men, according to a new study.

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Researchers say the combination of testosterone and progestin, a synthetic form of the hormone progesterone, worked better and faster to suppress than testosterone alone, according to a study that will be published in the May issue of the Journal of Clinical Endocrinology & Metabolism.

"It is possible to suppress sperm output to concentrations that are comparable with reliable contraception in most, but not all men," said associate professor Peter Y. Liu of the University of Sydney, Australia, and lead author of the study, in a statement. "The rate of suppression is comparable to that achieved after a vasectomy."

The contraceptive effects of testosterone and progestin would be reversible, according to researchers.

Testosterone and other male hormones act as contraceptives by creating negative feedback in a man’s reproductive system, suppressing the production of sperm, according to the study. Researchers analyzed all published male hormonal contraceptive studies from 1990 to 2006 with durations of at least three months.

In a total of 30 studies, 1,756 men between the ages of 18 and 51 were treated with different preparations of testosterone, with or without progestin.

While testosterone is the primary male sex hormone, progestin is used in combination with estrogen for female contraception and occurs naturally in females as progesterone.

"Progestin co-administered with testosterone increased both the rate and extent of suppression," Liu said. "It also may make long-term hormonal contraception safer by reducing the dose of testosterone needed for maintenance contraception."

The researchers also found that Caucasian men suppressed sperm output faster, but not as completely, as non-Caucasians. Younger men with lower natural testosterone levels had faster suppression, but the differences were relatively small, the study found.

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