D3 because this is: the active form of D & a large minority of people do not convert D or D2 (the prescription form of D) into D3 reliability. However, the only way to know is to measure serum D3 concentration, not just D or D2. Beware that many OTC brands do not work well; I have seen great success with an OTC brand containing 50,000 IU derived from sheep lanolin; usually taken about once weekly.
Answered 3/14/2015
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