1alpha,25-Dihydroxyvitamin D2 (1,25(OH)2D2; Paricalcitol) is a synthetic analog of the active form of vitamin D. It is a secosteroid hormone that acts as a selective activator of the vitamin D receptor (VDR). Its chemical structure is characterized by the presence of hydroxyl groups at the 1α and 25 positions of the vitamin D2 (ergocalciferol) backbone. The primary and well-established application of 1,25(OH)2D2 is as an active pharmaceutical ingredient (API) for the treatment of secondary hyperparathyroidism (SHPT) associated with chronic kidney disease (CKD). It is marketed under the brand name Zemplar. Compared to calcitriol (1,25(OH)2D3), it demonstrates a more selective action on the parathyroid glands with less pronounced effects on intestinal calcium and phosphate absorption, potentially reducing the risk of hypercalcemia and hyperphosphatemia. As a vitamin D analog, it is also a critical biochemical tool in life science research for studying VDR signaling pathways, calcium homeostasis, and cell differentiation. It is used in various in vitro and in vivo models to investigate its effects on immune modulation, cancer biology, and cardiovascular health.