The short answer is Not D2.
There are two forms of vitamin D you can purchase as supplements, vitamin D2 (ergocalciferol, Drisdol the RX form of D) and D3 (cholecalciferol, no RX version paid for by insurance).
D2 comes from plants. D3 is made by mammals from cholesterol with the assistance of UVB radiation and heat. The biological half life of D2 in humans has been determined to be about 3-5 days with complete elimination occuring in 1-2 weeks. The biological half life of D3 as determined by submarine mariners is about 10 weeks. There has been no studies looking at how long it takes to deplete a human to zero D3, although I have seen such patients. The decay of vitamin D or conversion to an inactive form is not a linear process so vitamin D does not follow normal pharmacokinetics.
Normal pharmacokinetics state that in 4 half-lives you reach steady state at a given dose or you reach near complete elimination in the absence of drug. Vitamin D2 would have to be dosed about every 3 days for there to be any sort of steady state because of its very short half life in humans. This is why patients replaced with vitamin D2 once a week or less frequently see variable increases in their vitamin D levels. In general weekly or monthly dosing of D2 will not produce significant rises in vitamin D levels or vitamin D effect. A recent meta-analysis on the effects of vitamin D on bone confirms the inefficacy of vitamin D as it is commonly prescribed. Vitamin D2 to be effective must be dosed about every 3 days. Unfortuanately, there is only one dose of D2 readily available and that is 50,000 IU. That is like trying to do eye surgery with a screw driver and hammer. It is the wrong tool for the job.
Vitamin D3 replacement at a given dose will reach steady state in 10 weeks the same as its half-life in submariners. The long half life of vitamin D3 allows for missed doses without a tremendous drop in blood level. It allows for make up doses and weekly or monthly dosing while still maintaining a steady blood level. Moreover, vitamin D3 is available over the counter at very low cost. Carlson Labs is a reputable manufacturer in business since the mid 1970s. A year’s supply of vitamin D3 gel caps by Carlson Labs costs about 25 dollars. The copay for a Drisdol (D2) prescription is likely to be as much for every month’s supply. You can get Carlson Labs vitamin D at many online retailers for very low cost including shipping.
So we have reviewed, pharmacokinetics, efficacy, convenience, and cost all coming down on the side of D3. The final issue is blood testing. Vitamin D2 is only reliably detectable by two current methods of measurement, HPLC dual mass spec and the Diasorin immunoassay. Mayo Clinic performs the mass spec technique with reliable precision but the prescision and reliability of other labs performing this technique is variable. (see my blog Vitamin D Testing Errors Continue) If you test your blood using a method that does not reliably detect D2 and your are taking D2, then you are not going to see a rise in your D level and this could be dangerous. LabCorp uses the Diasorin method to measure vitamin D capturing both D2 and D3 as a single total value. If you are not taking vitamin D2 then there is no concern over the D2 measurement issue only the accuracy of testing.
I hope this clears up all the confusion about D2 and D3. Happy supplementation.