The "protective" influence of cigarette smoking on Alzheimer's and Parkinson's diseases. Quagmire or opportunity for neuroepidemiology?

Department of Neurology, West Virginia University School of Medicine, Morgantown, USA.

The negative association between smoking and both Alzheimer's (AD) and Parkinson's (PD) diseases is a consistent epidemiologic finding. This observation has prompted many investigators to conclude that cigarette smoking must be "biologically protective" against AD and PD. Rather than suggesting some as-yet-undefined pathogenetic clue, however, this negative association more likely is indicative of the under-appreciated influence of differential survival in epidemiologic studies.