A year-long international clinical trial has shown that a new injected male contraceptive is almost 100 per cent effective in lowering sperm counts to the point a pregnancy is all but impossible.

The hormone-based drug lowers a man's sperm count by acting on the brain's pituitary gland. During the global clinical trial, the injectable was effective in nearly 96 per cent of the 320 men taking part.

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The injections contained a long-acting form of progestogen, a hormone that has the effect of blocking sperm production controlled by the pituitary gland. Testosterone was added to counter-balance reductions in levels of the male hormone resulting from the treatment.

Over 75 per cent of men in the clinical trial said they were satisfied with the injection and would continue to use it if it was available.

"The study found it is possible to have a hormonal contraceptive for men that reduces the risk of unplanned pregnancies in the partners of men who use it," said Dr. Mario Festin from the World Health Organization (WHO) who authored the study.

But since participants reported a high number of mostly mild adverse reactions to the injections, scientists might need to re-formulate the injections or experiment with different injection protocol before testing it again, said Dr. Festin.

"More research is needed to advance this concept to the point that it can be made widely available to men as a method of contraception. Although the injections were effective in reducing the rate of pregnancy, the combination of hormones needs to be studied more to consider a good balance between efficacy and safety."

The only existing contraceptive options for men are withdrawal, condoms or a vasectomy. Researchers, however, have been experimenting with a reversible hormonal contraceptive for men for four decades, said Dr. Festin. No safe, reliable product has ever been developed despite this extensive work, however.

The trial recruited 320 healthy men in Australia, Germany, the United Kingdom, Chile, India, Indonesia and Italy between the ages of 18 and 45 years old who were in stable, long-term monogamous relationships with healthy women.

Previous studies, however, show that men want a pill rather than an injection. 

Tagsinjected male contraceptive, birth control, pill, hormone-based drug, sperm count, pituitary gland, progestogen