GENEVA, Switzerland (CBS) — Birth control shots for men are an effective form of contraception but the side effects are a problem.

An international study just found they’re almost as effective as the pill for women. It was published in the Journal of Clinical Endocrinology & Metabolism.

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The injections work by using hormones to drastically lower sperm count.

Of the 266 married men who participated in the study, only 4 became fathers.

Sounds promising but the gender gap in contraceptive use may not close anytime soon.

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For one thing the pharmaceutical companies aren’t throwing a whole lot of money at the idea.

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“Their concern may be there’s a lack of profitability, maybe there is a question of gender bias,” says CBS medical contributor Dr. Tara Narula. “There’s a concern of regulatory hurdles. In addition, it’s not as easy to stop 1,500 sperm that are produced per second as opposed to one egg per month.”

Another concern are the side effects. Researchers actually stopped the study early due to mood changes and depression in some of the participants.

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Researchers say other side effects were acne, injection site pain, and increased libido.