An observational, uncontrolled study involving 156 consecutive patients with relapsing-remitting multiple sclerosis found that vitamin D3 supplementation may protect against relapse. Patients were under first-line immunomodulatory therapy and with initial vitamin D levels lower than 100 nmol/l (40 ng/ml). They were treated with 3010 IU vitamin D per day, on average. Relapses were determined for an average of 30 months during vitamin D and 30 months before supplementation. Researchers found a significant inverse relationship between the relapse incidence rate and vitamin D level and associated every 10 nmol increase in vitamin D level with a reduction in the relapse incidence rate. A plateau effect was observed after patients reached vitamin D levels of 110 nmol/l. This study suggests that vitamin D levels may be inversely associated with relapse rate for multiple sclerosis patients.