A paper published Feb. 14 in the journal Nature Communications, presents a novel approach to male contraception that looks promising in mice. Researchers tested a compound that blocks an enzyme sperm need to swim, suggesting a path to a fast-acting, temporary form of contraception. (Though tested in mice, many species including human males have the same enzyme.)

The drug stopped sperm from swimming, slowing their fast-beating tails to a twitch. In humans, this could mean they would never make it out of the vaginal canal past the cervix into the uterus. Further research showed that it was fast-acting, taking about fifteen minutes to have an effect. And it was temporary — it stayed in the system for several hours.

In those hours, the male and female mice in the study had plenty of sex. Within 2.5 hours after getting the drug, it was100% effective at preventing pregnancies. Within 3.5 hours, it was 91% effective.