Perhaps the most interesting research linking smoking and MS came from a 2007 study in the journal Brain that explicitly links the onset of MS before the age of 16 to childhood exposure to secondhand smoke. The longer the child was exposed to secondhand smoke, the more likely they were to develop MS. In the study, 62 percent of the 129 MS patients had been exposed to their parents' secondhand cigarette smoke during childhood compared to only 45.1 percent of healthy children.

Nevertheless, there's some controversy about whether smoking later in life increases the chances of developing MS. One study published in the Journal of Clinical Epidemiology found that the chances of getting MS among early smokers (between the ages of 11 and 45) is nearly three times higher than the chances of getting MS among smokers who started smoking after age 45.

People who start smoking at younger ages tend to smoke for a longer period of time and smoke more, says Dr. Ascherio. But that doesn't mean that smoking at a later age doesn't increase the risk of MS. Smoking is associated with a higher risk of MS, regardless of when you start.

AMY PATUREL, M.S., M.P.H.

Neurology Now  May/June 2009