Smoking and Parkinson's disease - An age-dependent risk effect?

We studied the association between smoking and Parkinson's disease (PD) through a case-control study. Several studies have shown an inverse association between smoking and PD. This association has been interpreted as spurious by some investigators, and as real and causal by others. Several other studies did not confirm the inverse association. We included 193 prevalent cases of PD ascertained in five European prevalence surveys that followed a two-phase design of screening and clinical examination. Each case was matched by center, age (+/-2 years), and gender to three control subjects drawn from the same populations (N = 579). Information on smoking was obtained through direct or proxy interview. Overall, there was no association between ever smoking and PD (odds ratio = 1.1; p = 0.6). Analyses stratified by age showed that ever smoking was associated with a decreased risk of PD in the younger individuals (odds ratio = 0.4; p = 0.03) and with a significant trend of increasing risk with advancing age (p = 0.003). The risk of PD in relation to smoking is strongly modified by age; smoking may be protective in the younger cases but not in the older cases. This finding may explain the conflicting results from previous studies.