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.
Vitamin D is essential for helping the body absorb calcium and maintain healthy bones. It also plays roles in muscle, nerve, and immune function. In the UK, people are advised to consider taking 10 micrograms (µg) of vitamin D each day during the darker months, when sunlight is generally too weak for the skin to make enough of the vitamin.
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