In a 1st, trial finds vitamin D supplements may slow multiple sclerosis. But questions remain. (2025)

For the first time, a clinical trial has demonstrated that taking high doses of vitamin D may slow the progression of multiple sclerosis.

In general, taking too much vitamin D can be dangerous; however, the high doses in the new trial were taken in controlled conditions, under the supervision of medical staff, and by patients for whom it was deemed safe.

Multiple sclerosis (MS) is a chronic autoimmune disease that affects the brain and spinal cord. In those with the disease, immune cells mistakenly attack the protective layer around nerve fibers, causing lesions, or scar tissue, to form. This leads to symptoms such as muscle weakness, changes in vision, numbness and memory problems, which may either come and go or get worse over time.

There is no cure for MS. Existing drugs can help patients manage their symptoms, but these medications often have nasty side effects, such as an increased risk of infections and gastrointestinal issues.

Related: Twin study reveals signs of MS that might be detectable before symptoms

Studies suggest that vitamin D deficiency is a potential risk factor for MS. That's because vitamin D can reduce inflammation in the central nervous system in numerous ways — for instance, by inhibiting the release of chemical messengers from immune cells that trigger inflammatory responses. Thus, the theory is that having too little vitamin D may allow runaway inflammation to ensue.

The link between low vitamin D and multiple sclerosis was first identified in the 1960s. Yet, over the years, various clinical trials have tried and failed to show that taking high doses of vitamin D can help to reduce a patients' symptoms or disease progression.

Previous trials may have failed because they included too few participants and monitored patients for only short periods of time, such as a year, said Dr. Eric Thouvenot, co-lead of the new clinical trial and a neurologist at the University of Montpellier in France. Past trials also tested vitamin D supplementation in patients who were already taking drugs to manage their MS, he told Live Science. This made it harder to determine how effective vitamin D was, in and of itself.

To overcome these hurdles, Thouvenot and colleagues took a different approach in their new trial.

They looked at the effects of high-dose vitamin D supplementation in 303 patients with clinically isolated syndrome (CIS). CIS describes when patients have early signs that are suggestive of multiple sclerosis but they have not yet developed the full-fledged disease and don't meet the diagnostic criteria. Signs of CIS include inflammation and damage to the protective layer around nerve fibers.

Although not a given that CIS will progress into multiple sclerosis, this is the case for approximately 85% of MS patients.

The patients in the trial had yet to start taking other drugs for multiple sclerosis, so the researchers had a window in which they could directly investigate the impact of vitamin D supplementation on their disease progression.

During the trial, the team randomly assigned 156 of the patients to take a high-dose of a form of vitamin D called cholecalciferol. They took the supplement once every two weeks for two years, or until they showed signs of MS symptoms or new or growing lesions during medical visits. These visits occurred three months after starting supplementation, one year out and then two years out, and they involved brain and spinal cord scans and MS symptom assessments.

If disease activity was detected, patients would be removed from the trial and immediately prescribed disease-modifying drugs, meaning drugs that alleviate symptoms by targeting the root causes of MS.

A comparison group of 147 patients underwent the same protocol but took a dummy drug instead of vitamin D, according to a report of the trial published March 10 in the journal JAMA.

The dose the vitamin D group received was 100,000 international units, which is around 20 times more than what common drug store supplements contain per dose.

Overall, the researchers found that disease activity occurred in 60% of the vitamin D group within the two-year study window, compared with 74% in the placebo group. The time it took for symptoms to emerge was also significantly longer in the former group than the latter — about 432 days, compared with 224 days.

Taken together, these findings suggest that early supplementation with high-dose vitamin D may help slow the progression of CIS to multiple sclerosis.

This is "important research" and supports the use of high-dose vitamin D by patients with multiple sclerosis, said Dr. Klaus Schmierer, a professor of neurology at Queen Mary University of London who was not involved in the new trial. Many doctors already recommend such supplementation, Schmierer told Live Science in an email.

Many clinicians recommend that MS patients take between 4,000 and 5,000 international units of vitamin D each day. Any more than this can cause a condition called hypercalcemia, in which too much calcium builds up in the body, because vitamin D normally regulates the levels of this mineral in tissues. Hypercalcemia can weaken bones and potentially lead to kidney and heart damage.

Despite being over the recommended limit for vitamin D supplementation, no one in the new trial who received vitamin D developed hypercalcemia. That hinted that this high dose may be safe in this context, and notably, the participants took the supplement twice a month, rather than daily.

However, ultimately, more trials are needed to corroborate these findings.

Furthermore, the aim of this research would not be to replace current disease-modifying drugs for MS but rather to provide another therapeutic option for patients who otherwise don't have access to the drugs, for example because of financial reasons, Thouvenot emphasized.