Why would vitamin D be prescribed when vitamin D3 is available over-the-counter?
Let's review the known differences between vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol):
--D3 is the human form; D2 is the non-human form found in plants.
--Dose for dose, D3 is more effective at raising blood levels of 25-hydroxy vitamin D than D2. It requires roughly twice to 250% of the dose of D2 to match that of D3 (Trang H et al 1998).
--D2 blood levels don't yield long-term sustained levels of 25-hydroxy vitamin D as does D3. When examined as a 28-day area under the curve (AUC--a superior measure of biologic exposure), D3 yields better than a 300% increased potency compared to D2. This means that it requires around 50,000 units D2 to match the effects of 15,000 units D3 (Armas LA et al 2004).
--D2 has lower binding affinity for vitamin D-binding protein, compared to D3
--Mitochondrial vitamin D 25-hydroxylase converts D3 to the 25-hydroxylated form five times more rapidly than D2.
--As we age, the ability to metabolize D2 is dramatically reduced, while D3 is not subject to this phenomenon (Harris SS et al 2002).
From Armas LA, Hollis BW, Heaney RP 2004
While there are dissenters on this view, the bulk of evidence suggests that D2 is an inferior form of D3.
Then why is D2 prescribed by many doctors when the natural, human, and superior D3 is available over-the-counter?
You already know the answer: Much of your doctor's education did not come from scientific lectures nor from reading scientific studies. It came from the pretty drug representative in the waiting room who hands the doctor reprints of the "studies" performed by the drug industry to support the use of their drugs. There is no such nutritional supplement representative in the waiting room. This preference for the "drug" D2 over the supplement D3 also stems from the inherent preference of physicians for things they can control, whether or not there is proof of superiority.
In my view, there is absolutely no reason to take vitamin D2 over D3 except to enrich the drug industry.