What affects the risk of MS: what research shows

How to read this page

In plain words: A risk factor describes what happens on average in large groups; it does not explain why one person developed MS.

Background

Risk factor studies compare groups of people. They can show that something is linked with MS, but most cannot show that it causes MS.

A 2015 review examined 44 pooled analyses covering 416 studies and warned that many had weaknesses that cast doubt on their results. Only three links passed all of its checks: antibodies to Epstein-Barr virus, infectious mononucleosis (glandular fever) and smoking.^[1]

Many studies report odds, which compare the chance of developing MS with the chance of not developing it. A figure such as "38% higher odds" compares two groups of people. It does not mean a 38% chance of developing MS.

The strongest links: Epstein-Barr virus and smoking

In plain words: Epstein-Barr virus infection and smoking have the strongest evidence. Most people who catch the virus never develop MS.

Large studies in people

A study of more than 10 million young adults in the US military included 955 who were diagnosed with MS during their service. The risk of MS rose 32-fold after infection with Epstein-Barr virus (EBV), and did not rise after other viruses, including cytomegalovirus. The authors write that the findings suggest EBV as the leading cause of MS.^[2]

In the 2015 review, people with antibodies to one EBV protein had about 4.5 times the odds of MS, and people who had had infectious mononucleosis about twice the odds.^[1]

EBV is one of the most common human viruses and most people catch it during their lifetime, usually without symptoms. MS is far less common, so most people who carry the virus do not develop it.^[3]

Smoking: a pooled analysis of 14 studies found about 1.5 times the risk of MS in smokers, also when smoking was recorded before MS began. Whether smoking speeds up later progression was less certain.^[4]

EBV spreads between people, but MS itself is not contagious. See What is MS? →

Weight in youth, vitamin D and sunlight

In plain words: Obesity in adolescence and low vitamin D are linked with MS, and genetic studies support both links. This does not show that taking vitamin D prevents MS.

Studies in people and genetic analyses

In two large groups of US women, followed for 40 years in total, with 593 confirmed cases of MS, obesity at age 18 was linked with more than twice the risk of MS. Weight in adulthood showed no clear link.^[5]

A Swedish study of 1,571 people with MS and 3,371 without found about twice the odds of MS in people with a body mass index above 27 at age 20, compared with people of normal weight.^[6]

Genetic analyses use inherited differences to reduce the effect of lifestyle and other factors. One found 41% higher odds of MS for each standard deviation, a statistical measure of spread, of higher genetically determined body mass index. A later review counts body mass index in childhood and adolescence among the credible risk factors.^{[7], [8]}

Among white US military personnel, higher vitamin D levels in the blood were linked with a lower risk of MS; no significant link was found among Black and Hispanic personnel. A genetic analysis found about twice the odds of MS for each standard deviation of lower genetically determined vitamin D level, measured on a logarithmic scale.^{[9], [10]}

In Australia, people with more sun exposure over their lives had a lower risk of a first episode of myelin damage.^[11]

What vitamin D does in people who already have MS is covered in the supplements and diets guide. →

Genes and family

In plain words: MS is not passed on by one gene. Close relatives have a higher risk, but the great majority never develop MS.

Genetic and registry studies

A study of 47,429 people with MS and 68,374 without mapped 233 genetic variants linked with MS. The genes it points to involve many parts of the immune system.^[12]

A Swedish registry study of 28,396 people with MS estimated the risk for their relatives, after adjusting for age: about 2.5% for a brother or sister, about 2% for a child and about 17% for an identical twin, the least precise of the three. In the general Swedish population, about 0.19% of people had MS.^[13]

So even most identical twins of people with MS did not develop it, which shows that genes are only part of the picture.^[13]

Under investigation: exposures at work

In plain words: A Swedish study links workplace exposure to petroleum-based lubricants with higher odds of MS. It shows an association and has not been repeated elsewhere.

One study, not yet repeated

The study appeared online in Neurology on 4 November 2026. It compared 2,872 people newly diagnosed with MS between 2005 and 2015 with 6,122 people without MS, chosen at random from the Swedish population register.^[14]

Lubricants here means products such as engine oil, hydraulic oil and metal-cutting fluids. Exposure at work was reported by 8.7% of people with MS and 6.5% of people without MS. Odds of MS were 38% higher in those exposed (odds ratio 1.38, 95% CI 1.16 to 1.65). Counting only people exposed at least seven years before their first symptom, they were 55% higher (odds ratio 1.55, 95% CI 1.27 to 1.90).^{[14], [15]}

The link was stronger in smokers and in people without the gene variant HLA-A*02:01. The figure of "more than 33 times", quoted in the news release, compares people with four features at once (exposure, smoking, a higher-risk gene variant and no lower-risk variant) with people who had none of them. That group held only 16 people with MS and 2 without, so the estimate is very uncertain.^{[14], [15]}

People reported their own exposure in a questionnaire, so it may have been misremembered, and other exposures at work may have played a part. The authors say the findings need to be repeated in other populations.^{[14], [15]}

The same Swedish research group reported in 2018 that exposure to organic solvents was linked with 1.5 times the odds of MS. They propose that irritation of the lungs may be the connection; this remains a hypothesis.^[16]

What the evidence does not support

In plain words: Reviews have not found that routine vaccines cause MS. A link with dental amalgam has not been shown.

Reviews of studies in people

A 2017 systematic review found no change in the risk of MS after vaccination against hepatitis B, human papillomavirus, seasonal flu, measles-mumps-rubella, tetanus, BCG, polio or diphtheria, among others. An earlier pooled analysis reached a similar view.^{[17], [18]}

A 2025 review of childhood factors found no substantial evidence of a clear link between vaccination and MS. Signals for the rubella and diphtheria vaccines rested on limited evidence from studies with serious flaws.^[19]

In people who already had MS, a study of 643 people with a relapse found that vaccination did not raise the short-term risk of relapse. The figures covered tetanus, hepatitis B and flu vaccines.^[20]

Dental amalgam: a 2007 pooled analysis found a slight increase that was not statistically significant. The authors asked for better studies before ruling a link out.^[21]

What it means for you today

In plain words: If you live with MS, nothing on this page means that you caused it. These studies concern the risk of developing MS, not how to treat it.

Summary of the evidence

The studies here describe groups. They cannot say why one particular person developed MS.

None of them shows that changing one factor prevents MS, and a single study is not a reason to change jobs. If you are exposed to chemicals at work and are worried, you can raise it with your doctor or occupational health service.

If you take treatment for MS, ask your neurologist about the timing of vaccines. Never stop or change prescribed treatment on your own.