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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.”