Researchers are continually on the hunt for a male contraceptive that could be the equivalent to the pill taken by women, and new research in Japan suggests we might have finally gotten there.

A study on mice by researchers from Osaka University has discovered a way of making male mice infertile temporarily by blocking a specific protein in the animals’ sperm – a protein that’s also found in human sperm.

“It is important that we find an effective and reversible contraceptive option to allow men more control over their own reproductive futures,” said Masahito Ikawa, the lead researcher from Osaka University’s Research Institute for Microbial Diseases, as reported by Amy Norton at HealthDay. “The findings of this study may be a key step to giving men that control.”

The protein blocked in the study, called calcineurin, was suppressed in mice by the use of two chemical inhibitors, producing temporarily infertile animals. The male mice with inhibited calcineurin still had sex with female mice, but none of the females became pregnant. The treatment took four to five days to render the males infertile, and a week after the animals discontinued taking the calcineurin inhibitors, their fertility returned.

While the treatment has not yet been reproduced in human males, there’s good reason to think it could work as a kind of male contraceptive pill. The chemical inhibitors used to suppress the male mice’s calcineurin proteins – cyclosporine A (aka CsA) and FK506 (aka tacrolimus) – are already available in existing human medicines, where they’re used as immunosuppressant drugs to help organ transplant recipients lower the risk that their bodies will reject new organs from donors.

While the researchers say the medicines shouldn’t be taken for contraceptive purposes in their current forms, the drugs’ newfound abilities to affect sperm could help research efforts towards the development of a male pill.

“Considering these results in mice, sperm calcineurin may be a target for reversible and rapidly acting human male contraceptives,” the researchers write in their paper, which is published in Science.

Unlike women, who have a range of contraceptive options open to them, men are faced with either using condoms or undergoing surgery to try to prevent pregnancy when having sex.

“Existing male contraceptives don’t come close to filling the need,” said Aaron Hamlin, executive director of the Male Contraception Initiative in the US, who was not involved with the research. “Condoms have a real-world annual pregnancy rate of 18 percent – about a dice roll for the average person.”