A drug commonly used to treat Type 2 diabetes may prevent cancer, researchers have said.
Metformin is currently licensed for treating diabetes
The team studied cancer rates among 6,000 people with diabetes and found those who took metformin cut their risk of all tumour types by more than 25%.
A larger study to verify the University of Dundee team's findings, published online at bmj.com, will begin soon.
Ultimately, metformin might be used as an anti-cancer drug, the researchers said at Diabetes UK's annual meeting.
Professor Dario Alessi and colleagues looked at the rates of cancers recorded among about 6,000 people with diabetes, half of whom were taking metformin and half of whom were not.
They found the metformin group had far fewer cancers than the other group - in the order of 20-40%.
They believe that the drug protects against cancer by switching on an enzyme in the body called AMPK.
This enzyme tricks cancer cells into thinking that they have insufficient sources of energy to grow.
Professor Andrew Morris said: "Metformin has been used to treat people with Type 2 diabetes in the United Kingdom since 1957.
"Despite being the most widely used drug for people with the condition, it is only recently that we have learnt how it works. The possibility that it may have beneficial effects beyond diabetes control is very exciting indeed."
Professor Alessi said that because the drug was already on the market it could cut out 10-15 years of testing that is usually associated with medical breakthroughs before a drug becomes available.
"If the trials confirm our findings, metformin could be used for cancer within a year or so.
"One needs to do many more clinical investigations, but it might be useful for people who have already had tumours removed, such as breast or colon cancers, to stop cancer recurring."
He said that although the drug, by its nature as a diabetes medication, lowers blood sugar, this might not be too much of a problem to people without diabetes.
"It's a remarkably well tolerated drug," he said.
Amanda Vezey, care advisor at Diabetes UK, said: "This is a very interesting study undertaken by researchers well known in the field of diabetes.
"However, it is in its very early stages and we would need to wait for the results of the larger scale study before we can draw any firm conclusions.
"We'll be following the progress of the next study with interest."
Professor John Toy, Medical Director at Cancer Research UK, said: "This is a very interesting early finding, but as the authors themselves say, further work is needed to confirm whether this is correct or not.
"We know that metformin affects the levels of insulin-like growth factor (IGF-1). Cancer Research UK scientists are working on this area as high levels of IGF-1 appear to confirm an increased risk of certain cancers including prostate, colon and breast."