There has been growing interest in determining environmental risk factors that may play a role in the development or progression of multiple sclerosis (MS). Epidemiological evidence and data from human and animal studies have shown an association between low serum vitamin D levels and an increased incidence of MS and that supplementation with vitamin D may protect against MS development and/or disease relapses. The most appropriate vitamin D dosage for patients with MS is unclear, but investigator shave proposed that serum vitamin D concentrations between 75 and 100 nmol/L (30-40 ng/mL) are optimal to achieve favor able clinical outcomes. Vitamin D supplemented in doses up to 3000 International Units (IU) daily may be necessary to achieve these levels in many patients, and doses of 500 to 800 IU daily appear to be necessary to maintain desired serum vitamin D levels.Short-term supplementation with doses up to 40 000 IU daily has been found to be safe. However, larger and longer clinical studies are needed to assess whether a true relationship exists between serum vitamin D concentrations and MS and to determine a safe and effective amount of vitamin D supplementation.