The difficult part? Men make a thousand sperm a second! To be an effective contraceptive, sperm count has to go down to zero!
Michael Lehmann has been involved in five testosterone based clinical trials. He's taken daily pills, monthly injections, a cream he rubbed on his shoulder, and even had an implant.
But testosterone could increase the risk of heart disease and prostate cancer. That's why Doctor John Amory is blocking vitamin A in the testes, which in turn blocks the development of sperm.
He says tests in mice show it works 100% of the time. But still, some doctors are skeptical.
While it may take time for men to get used to it, choosing when to have or not have a baby could be in their hands.
Researchers are also testing a drug that interferes with a protein, critical for sperm production and they say it's reversible. When mice were taken off the treatment, they became fertile again.