BIRTH control shots for men are effective at stopping pregnancy in female partners, but more research needs to be done to reduce the risk of side effects, according to new research.

The prospective study, carried out by the World Health Organization in Geneva, Switzerland, tested the safety and effectiveness of injectable contraceptives in 320 healthy men ages 18 to 45.

Participants had all been 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.

To suppress their sperm counts the men were given injections of 200 milligrams of a long-acting progestogen called norethisterone enanthate (NET-EN) and 1,000 milligrams of a long-acting androgen called testosterone undecanoate (TU).

The men received two injections every eight weeks for a period of up to 26 weeks.

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.

Once the sperm count was suppressed to less than 1 million/ml in two consecutive tests, the couples were asked to rely only on the injections for birth control.

During this second period, the men continued to receive the injections, this time every eight weeks for up to 56 weeks.

Semen samples were also taken every eight weeks to ensure the sperm counts remained at a low level.
After participants stopped receiving the injections the team continued to monitor the sperm counts to see how quickly they returned to normal.

The test results showed that the injections were effective in reducing the sperm count to 1 million/ml or less within 24 weeks in 274 of the participants, and was effective as a contraceptive method in nearly 96 percent of continuing users, with only four pregnancies occurring.

However there were also 1,491 reported side events, with common side effects including depression and other mood disorders as well as injection site pain, muscle pain, increased libido and acne.

Thirty-nine percent were found to be unrelated to the contraceptive injections, but due to these side effects new participants were stopped from enrolling in the study in 2011.

Although 20 men did drop out of the study due to the injection's adverse effects, more than 75 percent of the men still reported at the end of the study that they would be willing to use this method of contraception to prevent pregnancy.

Previous smaller studies have also shown the efficacy of the hormone injections for men, and the researchers are now continuing their work on the perfect combination of hormones in order to reduce the risk of experiencing side effects but whilst still providing an effective contraception option for men.

The results can be found published online in the the Endocrine Society's Journal of Clinical Endocrinology & Metabolism.