Routine hormone hip injections in 320 men dramatically reduced their sperm counts and prevented pregnancies during a year-long, early-phase trial, researchers reported Thursday.
The findings pointed to a hormonal male contraceptive in future, but according to the data, there were also side effects, such as acne and mood swings, which showed much work was needed before such a birth control method was realised.
According to Mario Festin, a medical officer with the department of reproductive health and research at the World Health Organization, told the Chicago Tribune, "A male hormonal contraceptive is possible."
He said, "We have to continue searching for or investigating the right drugs, and their combinations, with the highest efficacy and safety, and acceptability, with the least side effects."
In earlier work, it had been demonstrated that high doses of testosterone could rub out sperm count in men, but there were worrying side effects. Reducing the dosage level with a steroid hormone called progestogen, which activated the progesterone receptor, could possibly help resolve the issue.
In the new study, published in the Journal of Clinical Endocrinology and Metabolism, men aged 18 to 45 received hip injections containing 200mg of progestogen and 1000mg of testosterone, every eight weeks over the course of a 24 week ''suppression'' phase.
Semen samples were taken from the men at weeks eight and 12 of the study and then every two weeks until they met the criteria for the next phase. The couples were instructed to use other non-hormonal birth control methods during this time.
All participants were in monogamous relationships with female partners between the ages of 18 and 38 for at least a year, and all men had a normal sperm count at the start of the study.
When the sperm count was suppressed to under 1 million/ml in two consecutive tests, the couples were asked to rely only on the injections for birth control.
In the second phase the men received the injections, every eight weeks for up to 56 weeks.
To ensure that sperm counts remained at a low level, semen samples were also taken every eight weeks.
After the injections were stopped the team continued to monitor the sperm counts to see how quickly they returned to normal.
The test results revealed that injections effectively reduced sperm count to 1 million / ml or less within 24 weeks in 274 of the participants.
It proved effective as a contraceptive method in nearly 96 per cent of continuing users, with only four reported pregnancies.