To the Editor: In their study on aspirin, nonsteroidal anti-inflammatory drugs, and the risk of colorectal cancer, Dr Chan and colleagues1 conclude that regular, long-term aspirin use reduces risk of colorectal cancer. Although the study shows that long-term users of aspirin have a lower incidence of colorectal cancer, aspirin itself may not be the cause of this observation. The situation may be analogous to earlier results of the Nurses' Health Study,2 in which the apparent reduced cardiac and mortality risks from estrogen use may have been artifacts of confounding or selection bias, based on results of a subsequent randomized controlled trial.3
Vachon GC. Long-term Use of Aspirin and Risk of Colorectal Cancer. JAMA. 2005;294(24):3090–3091. doi:10.1001/jama.294.24.3090-b
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