An Australian scientist has made a discovery that could lead to a male contraceptive pill.
It will be at least 10 years before people can use it, says Dr Sabatino Ventura, but his aim is to develop a pill that temporarily prevents sperm from leaving a man's body.
So far he has managed to block sperm in mice, but only through a permanent genetic modification that leaves them without two specific proteins.
The next step is to develop a chemical that achieves the same affect, says Dr Ventura, who has been working on his project on-and-off for the past 25 years.
"There are two proteins that we need to block pharmacologically in the same way as we have blocked them genetically in the mice."
There is already an approved drug to block the one protein, but the other must be developed just about from scratch.
"If everything works well it will take about 10 years before people can use it."
The idea is that the reproductive system will revert to normal once the medication is stopped.
The laboratory tests proved that the sexual behaviour of the mice did not change.
"It all looked very normal, but there were no pregnancies," said Dr Ventura of Monash University in Melbourne.
"In a vasectomy the sperm are still normal. They just can't get transported to where where they need to go to be ejaculated out of the body. That's what we are trying to do chemically.
"The beauty of our strategy is that it is non-hormonal and so it won't affect sexual behaviour or the development of masculinity in men.
"It also does not affect the sperm, so there is no chance of it causing genetic defects in offspring."
But it could have a great impact.
"In the past year there were 60 to 80 million unwanted pregnancies around the world," said Dr Ventura.