Take home message: Keep your vit. D levels above 50ng/mL (or 125 nmol/L) but below 100 ng/mL (or 250 nmol/L) as levels above that you can have toxic side-effects. Note the two different units! Both are used and to avoid toxicity it is important to be aware what units your test results are given in so that you know what the normal range is. Make sure you are tested at least once per year if you are taking over 4000 IU per day. Short version: Vitamin D is more like a hormone than a vitamin and helps to regulate your immune system which is why it plays a major role in MS and other autoimmune diseases, heart disease, cancer, and bone health. For MS, simply maintaining a high-normal level helps reduce the number of attacks per year by almost the same amount as Copaxone or Interferons. It is also thought to prevent MS completely. Natural sunlight provides not only vit. D but other hormones and chemicals which synergistically work together for immune health such as ß-Endorphin, Calcitonin Gene-Related Peptide, Substance P, Adrenocorticotropic Hormone, Melanocyte-Stimulating Hormone, and nitric oxide. And as such, ideally one would get vit. D from sunlight rather than supplements if possible though since it is estimated 80% of Americans are deficient, taking a supplement could be necessary. The current concensus is most people can take up to 4000 IU daily without risking toxicity. Above that get a yearly or biannual blood test. Remember your levels will naturally be lowest after the winter (May) and highest after the summer (September). There is a growing amount of evidence, though also some controversy, in both vit. D’s effectiveness at preventing MS and also helping patients already diagnosed with MS. Considering it is cheap (I estimate it costs me ~$16 per year, has low risks, and good results, I've added it to my daily regimine. I take 6000 IU per day and get a yearly blood draw to check my levels. Currently my Vit. D levels are 59ng/mL. Remember that taking too much vit. D can be toxic and is also unnecessary for proper immune function. FAQ: Can’t I get enough Vit. D from the Sun? Probably not. Do you live outside and wear no sunscreen like our ancestors? Thought not. Always the final answer will be to get tested. You might get enough sunscreen if you live near the equator, but just being “outdoorsy” is not enough usually.