Several studies have shown that smokers have a lower risk of developing Parkinson's disease. But the relationship between risk for the disease and smoking may not be so simple. New research published in Neurology shows that smoking for a greater number of years may reduce the risk of the disease, but smoking a larger number of cigarettes per day made no difference.

The study involved 305,468 AARP members age 50 to 71 who completed a survey on diet and lifestyle at the time and again about 10 years later. During that time, 1,662 of the people had developed Parkinson's disease, or about one-half of one percent.

Current smokers were 44 percent less likely to develop Parkinson's disease than people who had never smoked. People who had smoked in the past and quit were 22 percent less likely to develop Parkinson's than people who had never smoked.

People who smoked for 40 or more years were 46 percent less likely to develop Parkinson's disease than people who never smoked. Those who smoked for 30 to 39 years were 35 percent less likely to have the disease than nonsmokers. In contrast, those who smoked for one to nine years were only eight percent less likely to get the disease.

The risk of developing Parkinson's disease did not change based on how many cigarettes a person smoked per day.

Studies have shown that smoking is not associated with a slower progression of the disease once Parkinson's develops or a reduced risk of death, so he said there is no evidence to support the use of nicotine or other smoking-related chemicals in treating the disease.

"These results could guide the development of studies on various tobacco components with animal models to help understand the relationship between smoking and Parkinson's disease," said study author Honglei Chen, MD, PhD, of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C.

"Research to reveal the underlying chemicals and mechanisms is warranted; such studies may lead to a better understanding of the causes of Parkinson's disease. However, given the many adverse consequences of smoking, no one would suggest smoking in order to prevent Parkinson's disease."

Citation: Chen et al., 'Smoking duration, intensity, and risk of Parkinson disease', Neurology, March 2010; doi:10.1212/WNL.0b013e3181d55f38