There are two types of vitamin D -  D2 and D3. 

Clinical trials have shown that only vitamin D3 (cholecalciferol) is helpful against MS or other conditions that can be helped by vitamin D. This is the form made by the body in response to sunlight

Researchers have concluded that vitamin D3's immunomodulatory potency is equivalent to other currently used immunosuppressants against MS – yet without their side effects, which can be severe.

If taking vitamin D as a supplement, it should be taken in the form of vitamin D3 (cholecalciferol).