Routine hormone injections into the buttocks of 320 men dramatically shrunk their sperm counts and prevented pregnancies during a year-long, early-phase trial, researchers reported Thursday.

The findings suggest that a future hormonal male contraceptive may one day be possible. However, the data also revealed high rates of side effects, such as acne and mood swings, suggesting much more work is needed before such a birth control method is realized.

"A male hormonal contraceptive is possible," lead author Mario Festin, a medical officer with the department of reproductive health and research at the World Health Organization, told the Chicago Tribune. "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."

Earlier work showed that high doses of testosterone could rub out sperm counts in men, but they also raised concerning side effects. Cutting the dosage level with a steroid hormone known as progestogen, which activates the progesterone receptor, was a potential workaround.

In the new study, published in the Journal of Clinical Endocrinology and Metabolism, Dr. Festin and colleagues gave men, aged 18 to 45 injections into their bums once every eight weeks over the course of a 24 week “suppression” phase. The injections contained 200mg of progestogen and 1000mg of testosterone. By the end, nearly 96 percent of those remaining in the study (some dropped out) saw their sperm counts fall to less than 1 million per milliliter. (Normal levels can range from 15 to more than 200 million/mL.)

From there, 266 sperm-suppressed men moved into a “testing” phase for up to 56 weeks. All of the men were in monogamous, heterosexual relationships and, at this point of the trial, were asked to give up alternative forms of birth control. (Their female partners were also screened at the onset of the whole trial. They had to have no desire to have a child within a two-year time period but also be willing to accept a low risk of pregnancy.) The men kept getting shots every eight weeks and had their sperm-counts monitored.

During the testing phase, four women got pregnant and six men saw their sperm counts rebound. Counting the failure rate during the “suppression” phase, the overall failure rate of the hormone shots was 7.5 percent—i.e., the shots had a 92.5 percent success rate.

However, many of the men reported of side effects. Nearly half of the 320 men (46 percent) reported developing acne. Thirty-eight percent reported an increased sex-drive. Twenty-three percent had injection-site pain, and 17 percent had emotional disorders.

After the study was complete, five percent of men didn’t recover their sperm count levels in the year after their last injection.

Still, in the end more than 75 percent of participants said they’d be willing to use the contraception method in the future if it were available. The study’s authors concluded that the method was “relatively good,” but needed tweaking.

The National Health Service in England noted that condoms are still clearly the better bet, with a 98 percent effective rate when used properly.

Journal of Clinical Endocrinology and Metabolism, 2016. DOI: 10.1210/jc.2016-2141 (About DOIs).