, Oct. 8 -- A commonly used vitamin D supplement may have an unexpected effect on the body, with research suggesting that vitamin D2 can reduce levels of vitamin D3, the form naturally produced when the skin is exposed to sunlight.

Vitamin D2 and D3 are often considered interchangeable because both can improve overall vitamin D levels. However, evidence suggests vitamin D3 is generally more effective at raising and maintaining vitamin D levels in the blood.

The U.S. National Institutes of Health says vitamin D3 tends to increase vitamin D levels more and keep them elevated for longer than D2.

Vitamin D helps the body absorb calcium and maintain healthy bones. It also supports muscle, nerve and immune functions.

A 2025 study published ...