Several compounds are produced along the complex pathways of vitamin D3 metabolism, and synthetic analogs have been generated to improve kinetics and/or vitamin D receptor activation. These metabolites display different chemical properties with respect to the parental or native vitamin D3, i.e., cholecalciferol, which has been, so far, the supplement most employed in the treatment of vitamin D inadequacy. Hydrophilic properties of vitamin D3 derivatives facilitate their intestinal absorption and their manageability in the case of intoxication because of the shorter half-life. Calcidiol is a more hydrophilic compound than parental vitamin D3. Active vitamin D analogs, capable of binding the vitamin D receptor evoking vitamin D-related biological effects, are mandatorily employed in hypoparathyroidism and kidney failure with impaired 1α-hydroxylation. They have been shown to increase BMD, supposedly ameliorating calcium absorption and/or directly affecting bone cells, although their use in these conditions is jeopardized by the development of hypercalciuria and mild hypercalcemia. Further studies are needed to assess their overall safety and effectiveness in the long-term and new intermittent regimens, especially when combined with the most effective antifracture agents.