Injected male contraceptive 'effective for 96% of couples'
An injected male contraceptive has been shown to be almost 100% effective in a trial involving 350 men.
The hormone-based jab is designed to lower sperm counts by acting on the brain's pituitary gland.
Over a year-long trial, nearly 96% of couples relying on the injection to prevent unplanned pregnancies found it to be effective. During this time, only four pregnancies occurred among the men's partners.
However, researchers said more work was needed to address the treatment's reported side effects, which included depression and other mood disorders, muscle pain, acne and increased libido.
The side effects caused 20 men to drop out of the trial.
Dr Mario Festin, from the World Health Organisation in Geneva, Switzerland, said: "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.
"Our findings confirmed the efficacy of this contraceptive method previously seen in small studies."
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.
After an initial period during which couples used both the injections and other birth control methods, the men entered the study's "efficacy phase" and relied on the jabs alone.
Throughout the efficacy phase, which lasted up to a year, the men were given injections every two months.
In 274 men, the injection reduced sperm count to one million per millilitre or fewer within 24 weeks.
Scientists stopped enrolling new participants into the study in 2011 owing to the rate of reported side effects.
Of the 1,491 incidents, nearly 39% were found to be unrelated to the treatment. They included one suicide.
One man experienced an abnormally fast and irregular heartbeat when he stopped receiving the injections.
At the end of the trial, three-quarters of the men said they would be willing to continue using the contraceptive jab.
The results are reported in the Journal of Clinical Endocrinology & Metabolism, published by the Endocrine Society.
Dr Festin said: "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."
Chris Barratt, Professor of Reproductive Medicine at the University of Dundee, said: "This is high-quality research from a very experienced group of investigators, and as there has been no progress in male contraceptives for 40-plus years this is a very significant and welcome development.
"Additionally, the fact that the study reports relatively low side effects and good ease of use are real world developments. The study involved a reasonable number of patients so the results are likely to be robust."
Leading fertility expert Professor Allan Pacey, from the University of Sheffield, said the evidence showed the injections to be "extremely effective" but he was concerned about the side effects.
"For a male contraceptive to be accepted by men (or women) then it has to be well tolerated and not cause further problems," said Prof Pacey.
"For me, this is the major concern of this study.
"But, it is noteworthy that 75% of the men who took part in the trial would be willing to use this method of contraception again."