The world's largest clinical trial to see whether taking aspirin every day can stop cancer returning has been launched.
The study will recruit 11,000 patients who have recently had - or are having - treatment for bowel, breast, oesophagus, prostate or stomach cancer.
It will run at more than 100 centres across the UK and will last for up to 12 years.
The study will compare a group of people taking 300mg of aspirin daily, a group taking 100mg of aspirin daily and a group taking dummy drugs.
Aspirin is already proven to help prevent heart attacks and strokes in some people. Previous research has suggested it could also prevent some types of cancer.
The Add-Aspirin phase III trial is funded by Cancer Research UK and the National Institute for Health Research.
Its overall aim is to see whether taking aspirin every day for five years can stop or delay cancers that have been caught and treated at an early stage from coming back.
Professor Ruth Langley, chief investigator from the Medical Research Council Clinical Trials Unit at University College London, said: "There's been some interesting research suggesting that aspirin could delay or stop early-stage cancers coming back, but there's been no randomised trial to give clear proof.
"This trial aims to answer this question once and for all. If we find that aspirin does stop these cancers returning, it could change future treatment - providing a cheap and simple way to help stop cancer coming back and helping more people survive.
"But, unless you are on the trial, it's important not to start taking aspirin until we have the full results as aspirin isn't suitable for everyone, and it can have serious side-effects. Please speak to your oncologist or research nurse if you would like to join the Add-Aspirin trial."
Dr Fiona Reddington, Cancer Research UK's head of population research, said: "Aspirin's possible effects on cancer are fascinating and we hope this trial will give us a clear answer on whether or not the drug helps stop some cancers coming back.
"This trial is especially exciting as cancers that recur are often harder to treat so finding a cheap and effective way to prevent this is potentially game-changing for patients."