Question & Answer

Does taking daily Vitamin D prevent multiple sclerosis later in life?

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The Evidence

  • A lot of research suggests a link between MS and having low levels of vitamin D. However, it is not clear whether low vitamin D levels play a part in causing MS, or whether low levels might be as a result of MS.
  • Most of the studies we found look at either the association between vitamin D levels and developing MS (i.e. whether the two are linked), or they look at effects of people who already have MS taking Vitamin D supplements. These studies do not answer the question of whether vitamin D supplements can stop MS developing in the first place. The reason such studies don’t exist is mostly because it is very difficult to do large-scale prevention trials that would best answer this question.
  • We found one high-quality observational study that found that women who used supplemental vitamin D had a 40% lower risk of MS than women who did not use vitamin D supplements. However, this supplemental vitamin D intake was largely from multivitamins which contain other vitamins as well as vitamin D. Also, just because a study finds links or associations between two things, this doesn’t prove that one thing is caused by the other.
  • We found two randomised trials that explored the effects of vitamin D supplements in people who have a high risk of developing MS (this is a type of MS called  ‘clinically isolated syndrome where people who experience it may or may not go on to develop MS).
  • One was a very small pilot trial of acceptable quality with 30 people (aged 20-40) that reported supplemental vitamin D can reduce the risk of MS developing in this type of patient; however, the other trial was of high quality with 204 people (aged 18-65) and concluded that vitamin D supplements did not reduce the risk of MS.
  • The dosage of vitamin D supplements used in these two trials (50,000 IU vitamin D3 weekly or up to 10,000 IU of vitamin D3 daily by mouth) was shown to be safe.

Guidelines and recommendations

  • We found no guidelines specifically on vitamin D supplementation for preventing MS. However, the HSE, the WHO and the CDC recommend vitamin D supplementation for general health.

Things to Remember

  • Although the two trials we found showed that the dosage of vitamin D they used was safe, it can be harmful to ‘overdose’ on high levels of vitamin D.
  • Be wary of treatment effects and claims that are based on small studies of few people.
  • Just because a study finds links or associations between two things, this doesn’t prove that one is caused by the other
  • Ask yourself if the advantages outweigh the disadvantages.


  • Lead Researcher: Dr Elaine Toomey
  • Reviewed by: Dr. Paula Byrne, Senior post-doctoral researcher, iHealthFacts, Evidence Synthesis Ireland and Cochrane Ireland, College of Medicine, Nursing and Health Sciences, University of Galway.
  • Topic advisor: Prof. Francis Finucane, Professor of Medicine University of Galway, Consultant Endocrinologist at Galway University Hospital.
  • Public and Patient advisor: Anne Daly, Public and Patient Involvement in research (PPI) advisor, PPI Ignite, University of Galway.
  • Journalist Advisor: Dr. Claire O’Connell, PhD in cell biology, Masters in Science Communication. Contributor to The Irish Times, writing about health, science and innovation.

Conflict of Interest Statement:  The authors have no financial or other conflicts of interest for this health claim summary.