The existing options for male contraception are condoms, vasectomy, or abstinence.

Therefore, the job of preventing unwanted pregnancy often falls to women, who may take a daily birth control pill, implant an intrauterine device, wear vaginal rings, use a diaphragm – or, when all else fails, take the morning-after pill.

Scientists are making progress in expanding options for sperm producers. An article published on February 14 in the journal Nature Communications presents a new approach to male contraception that looks promising in mice. The researchers tested a compound that blocks an enzyme that sperm need to swim, offering a path to a fast-acting, temporary form of contraception. (Although tested in mice, many species, including human males, have the same enzyme.)

The compound’s potential value as a male contraceptive was discovered by accident. Five years ago, a graduate student in Buck’s lab wanted to test it on mice as a possible treatment for a visual disorder. But the student was afraid of mice, so she asked another postdoctoral fellow, Melanie Balbach, for help. Balbach agreed, provided she could also test what happened to the male mice’s sperm, since she knew the drug acted on an enzyme associated with male fertility.

Balbach presented the results to Buck and lab co-director Lonnie Levine at a lab meeting the following week. They were stunning: after the male mice were injected with the compound, their sperm did not move.

“Lonnie’s reaction was: “Wow! This means we can develop a male contraceptive,” Buck recalls, “And my reaction was: “Lonnie, that’s even better. We can have a male contraceptive on demand.”

The drug stopped sperm from swimming, slowing their fast tails to a twitch. In humans, this can mean they can never make it out of the vaginal canal through the cervix and into the uterus. Further research showed that it works quickly, taking about fifteen minutes to take effect. And it was temporary – it stayed in the system for several hours.

During these hours, male and female mice in the study had a lot of sex. Within 2.5 hours of taking the drug, it was 100% effective in preventing pregnancy. After 3.5 hours, the effectiveness reached 91%.

Some other experimental concepts, such as hormonal pills, gels, and injections for men, can take weeks to start working. Some can cause mood disorders, affect alcohol tolerance, or shrink the testicles. Because male contraceptives are designed for healthy men, “the tolerance for side effects is going to be very, very low,” says Eisenberg, “[the enhancement] has to be pretty specific without a lot of these off-target effects.”