Listerine kills the germs that cause bad breath. But will Listerine kill you as well?
In 1991, researchers associated mouthwash with a 40 percent to 60 percent increase in oropharyngeal cancer. [Cancer Research 1991;51:3044-3047]
But as pointed out by these researchers,
As a general principle, a null association becomes apparently positive if a confounding variable is incompletely ascertained...
In other words, something [like mouthwash] that is not really a risk can be made to appear as a risk if a study does not consider adequately all other competing risks.
Shapiro et al. demonstrate that if heavy alcohol intake and tobacco use were underreported by study subjects in the 1991 mouthwash study, then the confounding from these two risk factors would have produced the spurious relationship between mouthwash use and oropharyngeal cancer.
More important than whether mouthwash use is associated with cancer, are the other points made by Shapiro et al., including:
Elevated odds ratios of 1.4 and 1.6 [i.e., reported increases in risk of as much as 60 percent] can readily be accounted for by [underascertainment of confounding factors].
...when there is clear evidence of uncontrolled confounding due [to underascertainment], we argue that a causal inference for associations of low magnitude is not justified.
...there are many studies in which it is claimed that, simply because a confounder has been measured and adjusted for, it has been adequately controlled. That claim may not be tenable if there is material underascertainment of the confounder at issue. As illustrated here, such underascertainment may produce artifactual associations.
What's it all mean? You've gotta be EXTRA careful with weak association epidemiology (i.e. studies that report relative risks under 2.0 or increases in risk of less than 100 percent).
Now, can Listerine can kill the germs that cause bad epidemiology?
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