Aspirin may protect against cancer but it is too risky to carry out randomised controlled trials, according to a report to be published in the Lancet.
It is written by Professor Peter Elwood, University of Cardiff, and Dr Alison Gallagher, Northern Ireland Centre for Food and Health at the University of Ulster.
They report that although there is much documented evidence that aspirin is beneficial, side effects such as bleeding would put an ethical question mark over efforts to codify the effects.
They conclude that there should be more research into creating a safer form of the drug while further data is gathered from every other possible source.
Amongst the evidence cited by the report is the work done on the development of polyps in the colon or rectum that precede colorectal cancer.
Aspirin has been shown to reduce the risk of polyps recurring or new polyps forming, and that its effect appears to increase in the later stages of polyp formation.
There is as yet no proven ideal dose, although two studies have suggested 81mg a day while others suggest daily doses of 300mg or more.
Copyright Press Association 2009