Vitamin D2 can lower your D3. Here is what that does to your blood test.

The finding. Researchers at the University of Surrey pooled randomized controlled trials that gave people vitamin D2 (ergocalciferol) and measured vitamin D3 in their blood. Of 20 trials in the review, 11 had usable data. In the meta-analysis, published in Nutrition Reviews in September 2025, people given D2 ended with 18 nmol/L less 25(OH)D3 than controls by end-of-trial comparison, and 9 nmol/L less measured as change over the trial. That is roughly 4 to 7 ng/mL. ScienceDaily picked the paper up again on October 8, 2026, which is why it is circulating now.

What it does not show. The comparison was D2 against no supplement, not D2 against D3, and it measured only the D3 fraction. The authors propose that the body speeds up clearance of vitamin D once levels rise, but they call for more research before it changes advice. So this is not evidence that D2 leaves you worse off overall. The head-to-head evidence comes from an earlier 2012 meta-analysis in the American Journal of Clinical Nutrition, from the same Surrey group: D3 raised total 25(OH)D more than D2 when given as a large bolus dose, and the difference disappeared with daily dosing.

Your report usually shows one number. The vitamin D test is a 25-hydroxyvitamin D (25(OH)D) level, and most reports give it as a single total: D2 and D3 added together. A D3 drop of the size above sits inside that total, offset by whatever D2 you are adding. Some labs run a fractionated test by liquid chromatography-mass spectrometry (LC-MS/MS) that prints 25(OH)D2 and 25(OH)D3 on separate lines. If your report has two vitamin D lines plus a total, that is the version you have.

Some tests read D2 poorly. Many labs measure vitamin D with automated immunoassays rather than mass spectrometry. In a 2016 study in the Journal of Clinical Biochemistry and Nutrition, 59 healthy adults were tested on two common immunoassays (Siemens ADVIA Centaur and Roche Elecsys) and by LC-MS/MS, before and 30 days after a 200,000 IU dose of D2. The immunoassays picked up 45% and 41% of the 25(OH)D2 that mass spectrometry found. The authors concluded clinicians should expect these assays to underestimate vitamin D status in people taking D2. That is one study on two platforms, and assays are revised over time, so treat it as a reason to ask, not a verdict on your lab.

Who this is most likely to affect. High-dose prescription vitamin D in the US is often ergocalciferol, sold as 50,000 IU capsules, and some plant-based supplements and fortified foods use D2 as well. If you are on any of these, both effects point the same way: your D3 may be lower than it would have been, and an immunoassay may not count all the D2 that replaced it.

How often to test at all. The Endocrine Society’s 2024 guideline suggests against routine vitamin D testing in the general populations it reviewed, and it set no target blood level. Testing still happens for good reasons: diagnosed deficiency, malabsorption, bone disease, or checking a treatment course. In those cases the method matters more than usual.

What to do with your own results. If you are taking D2 and a retest came back lower than expected, check which form you are taking and look at your report for the method (often printed under the result as LC-MS/MS or immunoassay). Ask your doctor whether a fractionated LC-MS/MS test or a switch to D3 makes sense before raising the dose. If D2 was prescribed for a specific condition, speak to the prescriber before switching.