http://dailynews.yahoo.com/h/nm/20010628/hl/smoking_3.html

Thursday June 28 1:23 PM ET
By Emma Patten-Hitt, PhD

NEW YORK (Reuters Health) - Long-time smokers may face an increased risk of multiple sclerosis, according to researchers from Harvard University. They found that women who smoked a pack a day for 25 years or more were more likely than nonsmokers to develop the disease.

Multiple sclerosis (MS) is a disease of the central nervous system believed to involve an abnormal immune system attack on nerve cells. The disease can lead to vision changes, muscle weakness, coordination problems and other debilitating symptoms. It strikes women more often than men.

According to the Boston researchers, led by Dr. Miguel A. Hernan, smoking has been linked to other immune system-related diseases such as rheumatoid arthritis and lupus.

To investigate a possible link between smoking and MS, the researchers combined data from two ongoing studies of nearly 240,000 US female nurses. Their smoking history was taken at the beginning of each of the studies. Every 2 years after that, the participants answered another survey about their smoking status and health.

Women from one study were followed for 18 years, and those from the other were tracked for 6 years. During that time, the researchers identified 315 definite or probable cases of MS.

Compared with nonsmokers, the risk that current smokers would develop MS was increased by 60%. Former smokers had a 20% higher risk than women who had never smoked.

Hernan's team also found that the more a woman smoked, the more likely she was to develop MS. Nurses who smoked a pack per day for 1 to 9 years were at a 10% increased risk of developing MS. Those who smoked the same amount for 10 to 24 years were at a 50% increased risk, and those who smoked a pack a day for 25 years were 70% more likely than nonsmokers to develop MS.

The researchers report the findings in the July 1st issue of the American Journal of Epidemiology.

"It is not known why smoking is linked to MS," Hernan told Reuters Health. The explanations, he said, range from the fact that smokers are more likely to develop respiratory infections (which may increase the risk of MS) to the direct toxic damage that components of cigarette smoke inflict on the nervous system.

Hernan also pointed out that these results probably hold true for men, as well. "Although no data are available, it seems likely that the association between smoking and MS exists among men, too," he said.

"If smoking causes MS, this would be...an additional reason to avoid smoking," Hernan pointed out, while noting that the risk of cancer and heart disease are even stronger reasons.

In addition to underscoring the importance of not smoking, he added, "elucidating the link between smoking and MS may help us understand the causes and lead to therapeutic and preventive advances."

SOURCE: American Journal of Epidemiology 2001;154:69-74.