Widespread use of low-dose aspirin could lead to a 10 per cent drop in the number of deaths from some cancers, studies suggest.
Millions of Britons take a daily aspirin, which can stave off heart attacks and strokes by preventing blood clots.
But for every 17 lives saved by preventing cancer or heart attacks there would be two deaths caused by strokes and bleeding or ulcers in the gut.
Taking aspirin over a decade would have benefits for most people aged between 50 and 70, research has shown.
But to be used more widely, experts say there needs to be a way to identify patients who are at a high risk of bleeding.
Aspirin reduces inflammation, but little is known about how it prevents cancer or what would be the best dose.
Cancer Research UK is now spearheading an international effort to answer these vital questions.
Prof Jack Cuzick, of Queen Mary University London, said: “This is a wonderful opportunity to finally answer questions that stand in the way of aspirin being more widely used to cut cancer risk.
“By bringing together researchers from the lab right through to epidemiology it will help us to understand how aspirin prevents cancer and who will benefit most.”
Researchers from Harvard, Newcastle University and University College London will also explore why aspirin appears to only have a powerful effect on some cancers.
A Chinese study last year found a daily aspirin was especially effective against cancer starting in the digestive system.
The risk of liver and oesophageal cancers was cut by 47 per cent and cancers starting in the stomach, pancreas and bowel fell by 38, 34 and 24 per cent.
Cancers of the lung, blood and prostate fell by as much as a third. Cancer Research UK’s Dr Fiona Reddington said: “This team will have the unprecedented opportunity to understand the major role aspirin plays in preventing cancer.”
A separate UK trial testing aspirin on 11,000 cancer patients started last year.