Vitamin D supplements are not interchangeable, and the distinction between vitamin D₂ (ergocalciferol) and vitamin D₃ (cholecalciferol) has real clinical consequences. Although both forms can raise measured vitamin D levels, vitamin D₃ is more potent, more stable, longer lasting, and biologically identical to the hormone the human body naturally produces. In contrast, vitamin D₂ is less effective, clears more rapidly, and may even suppress circulating vitamin D₃, undermining