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.

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.

Vitamin D supplements generally contain one of two forms. Vitamin D2, also called ergocalciferol, is commonly produced from fungi or yeast exposed to ultraviolet light. Vitamin D3, or cholecalciferol, is the same form produced in human skin after exposure to ultraviolet B radiation from sunlight.

Traditionally, vitamin D3 in supplements has often come from lanolin obtained from sheep's wool. However, animal-free D3 made from sources such as lichen is also available, an important consideration for people following vegan or plant-based diets.

Once vitamin D enters the body, it goes through several processing steps. The liver converts it into 25-hydroxyvitamin D, the form doctors commonly measure in blood tests to estimate a person's vitamin D status. Vitamin D2 produces 25-hydroxyvitamin D2, while vitamin D3 produces 25-hydroxyvitamin D3.

It is this D3-derived form that researchers found could fall after people took vitamin D2.

The 2025 study, published in Nutrition Reviews by researchers from the University of Surrey, John Innes Centre and Quadram Institute Bioscience, brought together evidence from randomized controlled trials to examine this effect more closely.

Researchers reviewed 20 studies, with 11 providing data suitable for the main meta-analysis. Compared with people who did not receive vitamin D2, those taking D2 had significantly lower concentrations of 25-hydroxyvitamin D3. Depending on how the results were analyzed, the average reduction was roughly 9 to 18 nanomoles per liter.

Importantly, that does not mean vitamin D2 simply removes vitamin D from the body or that taking it is necessarily harmful. Vitamin D2 itself raises 25-hydroxyvitamin D2 and can contribute to total vitamin D status. Instead, the surprising finding is that increasing one form appears to coincide with a reduction in the other.

Emily Brown, PhD Research Fellow and Lead Researcher of the study from the University of Surrey's Nutrition, Exercise, Chronobiology & Sleep Discipline, said:

"Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK. However, we discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements. This study suggests that subject to personal considerations, vitamin D3 supplements may be more beneficial for most individuals over vitamin D2."