The IARC convened a Working Group of experts in Lyon, France, on April 2-8, 1997 to evaluate the cancer-preventive activity of four nonsteroidal anti-inflammatory drugs (aspirin, sulindac, piroxicam, and indomethacin). Epidemiological observational studies of aspirin that differed in design, location, population, and motivating hypothesis have consistently shown that regular use lowers the risk for colorectal cancer by up to 50%; however, the one randomized trial did not show protection by aspirin. Definite evidence of chemopreventive activity normally requires data from appropriately designed randomized trials. The strength of the scientific evidence that aspirin prevents colorectal cancer in humans was thus considered to be limited. In animal models, there was sufficient evidence that aspirin prevented against cancer. Aspirin and aspirin-like drugs may have an important adverse side effects, the most frequent of which are gastrointestinal disturbances ranging in severity from dyspepsia to peptic ulcer. Given the remaining uncertainties in the preventive effect and the risk of adverse side effects, detailed consideration of the total benefits and of toxicity will be required before widespread use of aspirin for the prevention of colorectal cancer can be recommended. Sulindac shows promise in reducing the number and size of adenomatous polyps in patients with familial adenomatous polyposis. Further research is required, however, to determine whether and to what extent the risk for colorectal cancer in such patients is reduced. In people without familial adenomatous polyposis, there is inadequate evidence that sulindac has cancer-preventive activity. Fewer data were available on piroxicam and indomethacin than on aspirin and sulindac. Although these drugs consistently prevent colorectal cancers in experimental animal models, the evidence that they prevent colorectal cancer in humans was considered to be inadequate. The results of the meeting, including recommendations for future research, will be published as Volume 1 of the IARC Handbooks of Cancer Prevention.

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