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
In the study by Chan et al, there was a greater percentage of nonwhite participants among the aspirin nonusers compared with the users, an important difference. It is not clear if the relative risks were adjusted for race, but when there are measured confounders there also are likely to be unmeasured confounders. This may have been the case with the estrogen cohort studies, in which differences in lifetime socioeconomic status may have contributed both to differences in cardiovascular mortality and the propensity to take estrogen (and presumably other medications).4 Similarly, aspirin users may have already been at lower risk of colon cancer because of early-life socioeconomic conditions or other unmeasured confounders. Chan et al ask us to disregard randomized controlled trials showing no benefit of aspirin because of differences in doses and treatment length. It is, however, the results of observational studies that should be accepted only with great caution. Their best use, with respect to therapeutics, is to identify interventions worthy of a randomized trial.
Financial Disclosures: None reported.
Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature
Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal
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