vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). While both forms serve similar functions in the body, there are differences in their sources, metabolism, and effectiveness.

So it's ideal to go for a D3 supplement. (You can read more about the difference between D2 and D3 in the FAQ section below.) Monte Swarup, MD, and board-certified OB/GYN noted a few other things ...

Vitamin D3 seemed more effective than D2 at raising this important blood ... As humans age, these metabolic differences make a very large difference in terms of effectiveness.