An injection for men appears to be just as effective at preventing pregnancy as the birth control pill, finds new research that could revolutionize contraception.

In testing in China, only one man in 100 fathered a child while on the injections, the study in the Journal of Clinical Endocrinology & Metabolism reports.

The contraceptive is a form of testosterone that is injected into the buttocks once a month. It works by temporarily blocking sperm production.

Chinese researchers injected 1,045 healthy Chinese men aged 20 to 45 years with a 500 mg of testosterone undecanoate in oil, once a month for 24 months. All of the study participants had had at least one child and all their female partners, aged 18 and 38 years, also had normal reproductive function.

They found the contraceptive was almost 99 per cent effective, with a failure rate of only 1.1 per 100 men.

There were no serious side effected reported in any of the men, unlike previous studies on hormonal male contraceptive. Some of the side effects of those formulations have been mood swings and a lowered sex drive.

In all but two of the men in this study, reproductive function returned to normal range within six months of stopping the injections.

"For couples who cannot, or prefer not to use only female-oriented contraception, options have been limited to vasectomy, condom and withdrawal," said Dr. Yi-Qun Gu, of the National Research Institute for Family Planning in Beijing, China.

"Our study shows a male hormonal contraceptive regimen may be a potential, novel and workable alternative."

It should be noted though that almost a third of the 1,045 men in the 30-month trial did not complete the study. No reason was given.

Gu says while the results of this trial are encouraging, more long-term testing needs to be done on the safety of the regimen, with a focus on cardiovascular, prostate and behavioral safety.

Gu said if further tests proved successful, the treatment could become widely available in five years.