While investigating immunosuppressant drugs that inhibit the enzyme calcineurin, researchers at Osaka University in Japan stumbled across the potential future of male birth control. But there are a few caveats, the biggest and most immediate being that the discovery was in mice, and mouse studies have a way of fizzing out long before a clinical human trial gets before the FDA. Still, because the drugs — which are approved for diseases like rheumatoid arthritis — suppress calcineurin, and calcineurin is found in the sperm of both mice and men, the Osaka University scientists argue it has contraceptive potential.

Mice missing a gene for a key part of the calcineurin protein couldn’t produce offspring, the researchers reported in the journal Science on Thursday. Investigating the knockout sperm in vitro, the scientists found that when calcineurin doesn’t properly function, sperm are too rigid to summon the final speed boost required to penetrate egg membranes. In the final step, the scientists to gave the immunosuppressant to regular mice, which, after a few weeks, stopped being able to impregnate females. A week after taking them off the drug, the male mice could once again get the females in the family way. Reversibility — being the line between contraception and sterilization — is, of course, critical.

Don’t bank on popping this any time soon, though. Closer to approval — about three years away, according to its proponents — is Vasalgel, a polymer injected into scrotal tubes called the vans deferens. The goal is akin to a reversible vasectomy, using the gel to clog up the path sperm take to enter ejaculate. When you’re ready to get on with baby-making, the gel can, in theory anyway, be flushed out with an injection of baking soda solution.

Photos via Flickr.com/David