Alfacalcidol is an active metabolite of Vitamin D, which performs important functions in regulation of the calcium balance and the bone metabolism. Alfacalcidol is Vitamin D-hormone analog which is activated by the enzyme 25-hydroxylase in the liver for systemic and in osteoblasts for local D-hormone actions. It possesses a unique pattern of pleiotropic effects on, e.g. gut, bone, pararthyroids, muscle and brain. Alfacalcidol is superior to plain vitamin D (cholecalciferol) because the final kidney activation of the latter is regulated by a negative feedback mechanism. (PMID:17438884,17668216)    From: DrugBank