Prostate cancer is the most common cancer in men
A drug already used in men with enlarged prostates seems to cut the risk of prostate cancer developing, a large international study has shown.
A four-year trial in more than 6,500 men found those who took dutasteride had a 23% lower risk of prostate cancer than those who took a dummy pill.
The men in the study, published in the New England Journal of Medicine, were all at high risk of the disease.
UK experts welcomed the results but said longer-term research was needed.
It is not the first time that this class of drugs has been found to have potential effects on the prevention of prostate cancer.
In 2003, researchers showed a similar drug, finasteride, cut the risk of prostate cancer by a quarter in healthy men but also found that the tumours that were detected were more aggressive.
Both drugs are prescribed in men who have an enlarged but benign prostate gland - an extremely common condition that happens with age and can cause symptoms such as frequent and painful urination.
The latest trial, which was funded by GlaxoSmithKline which makes dutasteride, looked at men aged between 50 and 75 years who were high risk for prostate cancer because they had high PSA scores - a blood test which may indicate disease - but had been found not to have prostate cancer.
They were treated with a daily dose of dutasteride or a dummy pill.
Over the study, prostate cancer was found in 659 of the men taking the drug and 858 of the men taking a placebo.
Among men with a family history of prostate cancer, the drug reduced the relative risk of a prostate cancer diagnosis by 31.4%.
The researchers believe that most of the tumours detected during the trial - which tended to be moderately aggressive - would have been present from the beginning but were too small to be detected.
They say this supports the idea that the drug shrinks early prostate tumours or prevents them from growing to a size at which they would be detected and need treatment.
Study leader Dr Gerald Andriole, chief of urologic surgery at Washington University School of Medicine, said the drug might potentially offer thousands of men a chance to reduce their risk of the disease.
"This means more men could avoid unnecessary treatment for prostate cancer along with the costs and harmful side effects that can occur with treatment."
The National Institute for Health and Clinical Excellence (NICE) are already looking into whether dutasteride should be used for prostate cancer prevention and are due to report later this year.
Martin Ledwick, Cancer Research UK's head information nurse, said the results in the high risk population were interesting.
But he added: "Further research to see if the effects are maintained in the longer term is an important next step."
Dr Helen Rippon, head of research management at The Prostate Cancer Charity, agreed: "Of course, we don't yet know what will happen to these men in the coming years and whether they will still go on to develop the disease and it will be many years before we know if the drug can provide any long-term benefit to men."