A new study looked at health records from around 55,000 UK men who had been prescribed 5-alpha-reductase inhibitors over an 11-year period.
A team led by the University of Edinburgh and UCL found the drugs were linked to an increase in the risk of developing type 2 diabetes of about one third.
It follows previous short-term studies which suggested the drugs, which include finasteride and dutasteride, might affect metabolism and could reduce the body’s response to insulin - an early sign of type 2 diabetes.
Men with enlarged prostates are commonly prescribed 5-alpha-reductase inhibitors that reduce the production of hormones called androgens.
They help treat symptoms such as reduced urinary flow.
The study authors stressed patients should continue taking the drugs, but said they may need extra health checks.
The findings suggest that in a population of 500 men on the treatment for 20 years, 16 extra cases of diabetes are likely to develop.
A similar effect was seen when the team repeated the study with health records from a group of Taiwanese men.
Prof Ruth Andrew, from the British Heart Foundation Centre for Cardiovascular Science at Edinburgh University, said: “These findings will be particularly important for health screening in older men who are already typically at a higher risk of type 2 diabetes.
“We will now continue our studies to better understand the long-term outcomes so we can better identify patients at greater risk.”
Laurence Stewart, consultant urologist at Spire Murrayfield Hospital, said: “These findings should not be a major concern for men taking 5-alpha-reductase inhibitor medications.
“As doctors, we may need to review the way we monitor our patients to make sure we are extra vigilant for early signs of diabetes.
“Anyone with concerns should speak to their GP or urologist for advice on alternative treatments.”
The research was published in the British Medical Journal and was funded by the Edinburgh and Lothians Health Foundation.
Researchers from the Universities of Dundee and Newcastle and National Cheng Kung University in Taiwan also contributed to the study.