Vitamin D deficiency affects persons of all ages. Common manifestations of vitamin D deficiency are symmetric low back pain, proximal muscle weakness, muscle aches, and throbbing bone pain elicited with pressure over the sternum or tibia. A 25-hydroxyvitamin D level should be obtained in patients with suspected vitamin D deficiency. Deficiency is defined as a serum 25-hydroxyvitamin D level of less than 20 ng per mL (50 nmol per L), and insufficiency is defined as a serum 25-hydroxyvitamin D level of 20 to 30 ng per mL (50 to 75 nmol per L). The goal of treatment is to normalize vitamin D levels to relieve symptoms and decrease the risk of fractures, falls, and other adverse health outcomes. To prevent vitamin D deficiency, the American Academy of Pediatrics recommends that infants and children receive at least 400 IU per day from diet and supplements. Evidence shows that vitamin D supplementation of at least 700 to 800 IU per day reduces fracture and fall rates in adults. In persons with vitamin D deficiency, treatment may include oral ergocalciferol (vitamin D2) at 50,000 IU per week for eight weeks. After vitamin D levels normalize, experts recommend maintenance dosages of cholecalciferol (vitamin D3) at 800 to 1,000 IU per day from dietary and supplemental sources.