The risk was increased for people whether they were smokers at the time they developed MS or were past smokers.
"This is one more reason for young people to avoid smoking," said study author Trond Riise, PhD, of the University of Bergen in Norway. "Hopefully, these results will help us learn more about what causes MS by looking at how smoking affects the onset of the disease."
The study examined 22,312 people living in Hordaland County, Norway, who were age 40 to 47 at the time of the study. Questionnaires were used to determine whether people had ever smoked and whether they had MS or other diseases.
Of the 22,312 people, 87 had MS. The smokers were 1.81 times more likely to develop MS than the non-smokers. Men who smoked were 2.75 times more likely to develop MS than men who never smoked. Women who smoked were 1.61 times more likely to develop MS than women who never smoked. Smoking increased the risk of developing MS for people at all education levels.
There was an average of 15 years from when the people with MS started smoking to when they developed the disease.
The cause of MS is unknown. But experts believe that a combination of genetic and environmental factors is responsible.
"Environmental factors, which could include infections or exposure to solvents, and lifestyle factors, such as smoking and diet, may cause the disease to develop in people who are genetically susceptible to it," said neurologist Gary Franklin, MD, MPH, of the University of Washington School of Public Health and Community Medicine in Seattle, who wrote an editorial accompanying the study. "Neither having the susceptible genes by themselves, nor just being exposed to the environmental factors alone, is enough to cause the disease."
MS is an autoimmune disease, which means that the person's own immune system attacks the central nervous system. Smoking has been linked with other diseases that operate through the immune system, such as rheumatoid arthritis.
Riise said there are several biological factors that could explain the increased risk of MS among smokers, such as the effect of smoking on the immune system and the central nervous system. He said the role of these mechanisms and of components of cigarette smoke, such as nicotine, in the development of MS needs to be explored further.
The American Academy of Neurology, an association of more than 18,000 neurologists and neuroscience professionals, is dedicated to improving patient care through education and research. A neurologist is a doctor with specialized training in diagnosing, treating and managing disorders of the brain and nervous system such as stroke, Alzheimer's disease, epilepsy, Parkinson's disease, autism and multiple sclerosis.
For more information about the American Academy of Neurology, visit its web site at www.aan.com.
Editor's Note: A Neurology "Patient Page" was developed to correspond to this study and contains additional background and resources for MS. A copy is available from Media Relations staff. After the embargo, the "Patient Page" is publicly accessible at www.neurology.org.