A drug commonly used to treat baldness appears to prevent some men from developing prostate cancer.
Prostate cancer is relatively common
Trials of the drug, finasteride, produced such promising results that they were stopped early so that it could be given to everybody taking part.
The drug appears to cut the risk of prostate cancer by 25%.
Lead researcher Dr Ian Thompson, of the University of Texas Health Sciences Center at San Antonio, told a news conference on Tuesday the drug had "extraordinary public health potential".
He said: "These are very important results.
This is the first intervention that has proved to reduce a man's risk of prostate cancer
"This is the first intervention that has proved to reduce a man's risk of prostate cancer."
The drug was developed to stop the benign enlargement of the prostate that comes with normal aging.
A lower-dose version called Propecia became best-selling
baldness drug when it was also found to stop hair loss.
The researchers believe the drug could potentially benefit many men, as prostate cancer is a relatively common form of the disease.
The drug was tested in a long-term trial of 18,882 men, who were given daily doses of finasteride, or a placebo over a seven year period.
Each volunteer was given an annual blood test and rectal examination to check for signs of prostate cancer.
And after seven years each agreed to have a tiny sample of prostate tissue removed for analysis.
The researchers found that 24% of the men given a placebo developed prostate cancer - but among the finasteride group the rate was just 18%.
The men in the finasteride group who did develop cancer were more likely to develop a more aggressive form of the disease.
Dr Scott Lucia, of the University of Colorado, who examined
the tissue samples, said it was possible - but unlikely - the drug somehow encouraged more aggressive tumours.
He said it was more likely that the drug shrank the prostate
so that the few tumours that could resist its effect became more apparent.
Or it was possible that the drug changed the appearance of the tumours that developed, making them look more
dangerous than they were.
However, Dr Peter Scardino of New York's Memorial Sloan-Kettering Cancer Centre, said: "It looks like Proscar prevented little tiny, insignificant cancers, but did nothing for
high-grade cancers or maybe even allowed them to become more common.
"That doesn't sound like a very good trade-off to me."
The drug was also associated with a loss of sex drive.
The research is published in the New England Journal of Medicine.