However, I also am going to present the science as I found it, which isn’t particularly kind to smokers in terms of risk for developing MS.
The Data Behind Smoking and Increased Risk for Multiple Sclerosis
Smokers Diagnosed Younger: In one study, people who ever smoked were about 5 times as likely to be diagnosed with MS before the median age of 33 as those who never smoked.
Ever vs. Never: A metanalysis was performed on six studies which examined the correlation between smoking cigarettes and developing MS, which showed that smokers are between 1.22 and 1.51 times as likely to develop MS as people who have never smoked. To put these numbers in more tangible terms, if a non-smoker has a 1 in 1,000 risk of developing MS, a smoker has a risk of between 1 in 819 to 1 in 662 of developing MS (if all other factors are equal).
“Dose” Matters: A Canadian study showed that smoking more than 40 cigarettes (2 packs) a day resulted in a risk for developing MS that was 5.5 times that of a non-smoker, while people smoking between 1 and 2 packs per day had about twice the risk. Another study showed that logging more than 25 pack years before diagnosis made a person 1.7 times (almost twice) as likely to develop MS than a person who did not smoke.
Note: “Pack years” are calculated by multiplying the number of pack equivalent smoked every day by the total number of years. This means that if someone smokes 1 pack per day for 10 years, they have smoked 10 pack years; if someone has smoked 2 packs per day for 5 years, they have also smoked 10 pack years, as has someone smoking half of a pack per day for 20 years.
Early Smoking Triples Risk: A recent study out of Johns Hopkins, led by Joseph Finkelstein and presented at the American Academy of Neurology’s 61st Annual Meeting in Seattle, April 25 to May 2, 2009, shows that people who start smoking before they turn 17 (labeled “early smokers”) were 2.7 times more likely to develop MS than people who never smoked.
Interestingly, in this study, people who initiated smoking at age 17 or later did not have an increased risk for the disease. More than 32 percent of the MS patients in the study were early smokers, compared to 19 percent of the people without MS.
Conversion from Clinically Isolated Syndrome to MS: One study followed 129 people who had experienced an episode of clinically isolated syndrome, a situation that is almost like MS, except it has only happened one time (typically, a diagnosis of MS[ requires “multiple” lesions and/or multiple clinical episodes or “relapses”). These patients were asked about their smoking status at the time that they experienced the clinically isolated syndrome, and followed for 36 months. During this time, 75% of the smokers had another relapse, which made the diagnosis of MS official. Only 51% of the non-smokers had a relapse during these three years, making it almost twice as likely that the smokers would develop MS after clinically isolated syndrome (within a three-year time period). In addition, the time until the relapse came was much shorter in the smokers than the non-smokers.
The Bottom Line
I have gotten several e-mails from readers who have run across stories in the media about the link between smoking and risk of developing MS. Usually, these are people who are very upset because one of their children smokes and some relative has MS, therefore the parents conclude that their sons or daughters should immediately get an MRI, as they are sure that they have “given themselves” multiple sclerosis by lighting up. I also imagine that if you have MS and ever smoked, that you might have feelings of regret and taking trips down the “what if I had never tried that first cigarette” path. Or you may never have smoked or smoked very little and know someone who has smoked a couple packs a day for the last 20 years who doesn’t have MS, which might make you feel a little bit angry about the whole situation (subconsciously, of course).
I have to reiterate something here – smoking may contribute to MS risk, but it does not cause MS directly. If that were the case, way, way more people would have MS, and people who never smoked would be much less likely to get it. Nobody yet knows what causes MS. It is probably a very complicated mix of ingredients that includes genetic factors, vitamin D exposure and metabolism, hormones, exposure to certain viruses – all which combine in just the right way to trip just the wrong switch in our immune systems to give a few of us multiple sclerosis.
Smoking probably messes with some of these factors. For instance, it is shown to decrease the absorption of vitamin D by 25% or more. It is also shown to put a strain on immune function by releasing huge amounts of free radicals into the body – this might mean that it skews the wrong cells to do the wrong things, or throws the “balance of power” in the immune system off. There are a number of other theories, which you can read about in How Smoking Makes Multiple Sclerosis Worse: Some Proposed “Mechanisms of Action”.
In the end, no one thing you did “gave” you MS. If you smoke and do not have MS, you can draw your own conclusions and do a smoking risk/benefit analysis yourself to determine if you are going to factor MS risk into a decision to quit or keep smoking. I guess I’m obligated to tell you that if you have MS and still smoke, quitting can have some benefits, which you can read about in Does Smoking Make MS Worse? Check out the facts, decide for yourself.