Differentiation between Ercalcidiol (25(OH)D 2 ) and 25(OH)D 3 is important for monitoring vitamin D therapy, as vitamin D 2 is the predominant prescription form. The half-life of Ercalcidiol is shorter than that of 25(OH)D 3 and it binds less well to the vitamin D binding protein, making it less potent and, therefore, required to be administered at much higher doses than vitamin D 3 . Some currently used assays have a diminished capacity to detect Ercalcidiol, which can lead to dangerous overdosing when attempting to monitor therapy with vitamin D 2 .