Vitamin D—available in forms such as ergocalciferol (D2), cholecalciferol (D3), and alfacalcidol—is a fat-soluble vitamin that helps your body absorb calcium and phosphorus. These nutrients are essential for building and maintaining strong, healthy bones. Getting the right balance of vitamin D, calcium, and phosphorus is important for preventing and treating bone-related conditions like rickets and osteomalacia. Vitamin D is naturally produced in the skin when it’s exposed to sunlight. However, factors like sunscreen use, protective clothing, limited sun exposure, darker skin tones, and aging can reduce how much vitamin D your body makes from the sun. Vitamin D is often combined with calcium to help treat or prevent boPne loss (osteoporosis). It can also be used along with other medications to manage low calcium or phosphate levels caused by certain medical conditions, such as hypoparathyroidism, pseudohypoparathyroidism, and familial hypophosphatemia. In people with kidney disease, vitamin D may help maintain normal calcium levels and support proper bone development. For breastfed infants—since breast milk typically contains low levels of vitamin D—vitamin D drops or supplements are usually recommended.