It depends, honestly. My endocrinologist told me OTC D3 absorbs better than prescription D2, so instead of doing a 50,000IU D2 pill every so often, he had me taking 20,000 IUs of D3 every day. Likewise when my lab work showed I needed folic acid or iron, they always had me buy it OTC. The only thing I ever got by prescription was B12, but that was because I was injecting it.