Yes, taking vitamin D2 can lower blood levels of vitamin D3, though that does not prove it harms long-term health. A ScienceDaily report from the University of Surrey highlights this finding. It matters for our Vitamin D Supplementation community because D2 and D3 are different forms. D3 is also the form your body makes with sunlight. The report says D3 appears better at raising and maintaining overall vitamin D levels. But lower D3 does not mean D2 has no benefit. A separate 2026 review combined results from 26 studies involving people taking D2. In these studies, people were assigned to treatment groups by chance. The provided report does not give their ages or total number. That review found a clear drop in parathyroid hormone, which helps control Calcium. It also found a small rise in blood Calcium. These changes show that D2 still has effects in the body. The report does not say how large the D3 drop was. Both forms can contribute to vitamin D levels. What remains unclear is whether their differences lead to different long-term health outcomes. A change in a blood test alone cannot settle that question. For members tracking Vitamin D (25-OH), the form on the label adds useful context. Recording D2 or D3 alongside dose and test dates can make community comparisons clearer. Sharing those details with your bloodwork helps others understand your routine, without treating one person's results as proof. What changes have you seen in your bloodwork while taking D2 or D3?