Figure 1: Chemical structures of vitamin D2 and D3.(A) Structure of vitamin D2, otherwise known as ergocalciferol. (B) Structure of vitamin D3, which may also be referred to as cholecalciferol. Chemically, the two forms of the vitamin are related in that they are both secosteroids. Secosteroids are steroids in which one of the double bonds in the steroid ring is broken. Structurally, vitamin D2 and vitamin D3 differ due to their side chains. The side chain of D2 contains a double bond between carbon positions 22 and 23, and a methyl group on carbon 24.