Scientists warn a popular vitamin D supplement may have a hidden downside

A common vitamin D supplement may have an unexpected effect inside the body. Research published in 2025 found that taking vitamin D2 can reduce levels of vitamin D3, the form humans naturally make when skin is exposed to sunlight.

That distinction matters because vitamin D2 and vitamin D3 are often treated as interchangeable sources of the same nutrient. Both can increase overall vitamin D status, but a growing body of evidence indicates that D3 is generally more effective at raising and maintaining the main form of vitamin D measured in the blood. Current guidance from the U.S. National Institutes of Health reflects that evidence, noting that D3 tends to raise vitamin D levels more and keep them elevated for longer than D2.