Groundbreaking work with lab mice has boosted hopes for a male contraceptive pill, researchers in the United States report.
A compound initially sketched as a candidate for blocking cancer has been found to stop sperm generation in mice, they say.
Once the drug was halted, the rodents recovered fertility and were able to sire perfectly healthy offspring.
"If you stop the drug, there's complete reversibility," said Martin Matzuk, director of the Center for Drug Discovery at Baylor College of Medicine in Houston, Texas.
The drug is known by its lab name JQ1, after a chemist, Jun Qi, who devised it, initially with the idea of disrupting a cancer-causing gene called BRD4.
But it has also been found to inhibit proteins called bromodomains, one of which, BRDT, plays a key role in generating sperm in the testes.
By latching onto BRDT, JQ1 reduces dramatically both the number and quality of sperm, effectively leaving the mice infertile.
One of the attractions of JQ1 is that it is a small molecule, meaning it can sneak through the physical barrier between blood vessels and specialised sperm-making cells in the testes. Large molecules cannot do this, a problem that has thwarted work in the past.
Male contraception is limited to the condom or vasectomy, so an easy-to-use reversible option is badly needed.
JQ1 is unlikely to be the desired outcome by itself "because it also binds other members of the bromodomain family," explained Matzuk.
"However, the data is proof of principle that BRDT is an excellent target for male contraception and provides us with useful information for future drug development."
The study appears in the US journal Cell.