A reduced risk for Parkinson's disease (PD) among cigarette smokers has been observed consistently during the past 30 years. Recent evidence suggests that caffeine may also be protective. Findings are presented regarding associations of PD with smoking, caffeine intake, and alcohol consumption from a case-control study conducted in western Washington State in 1992–2000.
Being a cigarette smoker was observed to reduce risk of PD. This study does not tell us if it's the smoking itself that's causing this, or some other trait of cigarette smokers because the studies weren't randomized.
