Association between cigarette smoking and Parkinson's disease: A meta-analysis

Objective: To evaluate the association of Parkinson's disease (PD) with smoking, and determine whether gender, source of controls, dose of smoking, and year of studies modify the observed effects of smoking on PD. Methods: Available publications between 1959 and 2014 from PubMed, ScienceDirect, Springer Link and Web of Science databases were searched and carefully selected. Relative risks RR of specific study were weighted to obtain a pooled RR estimate and its 95% confidence interval CI. Results: 61 case-control and 8 cohort studies were included. The pooled RR of PD was 0.59 (95% CI, 0.56-0.62) for ever smokers compared with never smokers. The stratified analyses indicated a somewhat greater impact of smoking on PD risk in cohort studies than in case-control studies, the protective effect was relatively significant in men more than in women and the inverse effect was slightly greater in hospital-based studies than in population-based studies. Furthermore, a significant inverse doseresponse relationship was observed for the number of pack-years smoked. The summary RR for those smoking more than 30 pack-years was 0.66 (95% CI, 0.49-0.88), and 0.39 (95% CI, 0.29-0.53) for those smoking less than 30 pack-years. Conclusion: The results demonstrated the inverse association between cigarette smoking and the risk of PD. We suggest that effective drugs for PD might be developed using chemical substances derived from tobacco or tobacco smoke. (C) 2015 Elsevier Ireland Ltd. All rights reserved.