Vitamin D deficiency is a re-emerging global health problem, which is primarily due to inadequate vitamin D synthesis in the skin. Supplement use is an effective measure to improve vitamin D status. However, some safety issues have to be considered, which are highlighted in this review article: The concept of vitamin D safety consists of two models, the safe tolerable upper intake level (UL) method, and the idea of adequate circulating 25-hydroxyvitamin D (25[OH]D) levels. Oral vitamin D intakes up to 250 μg/d have not been associated with harm. Hypercalcemia, the hallmark of vitamin D intoxication, may only occur if circulating 25(OH)D levels are consistently above 375-500 nmol/l. However, some observational studies indicate that already circulating 25(OH)D levels > 125 nmol/l are related to an increased morbidity and mortality risk. Therefore, the Institute of Medicine has set the UL for adults at 100 μg/d, and the adequate circulating 25(OH)D level at 50 to 125 nmol/l. In clinical practice, oral vitamin D dosing has to consider that the increment in circulating 25(OH)D depends on baseline 25(OH)D levels and the person's body weight. It is reasonable to assess 25(OH)D before and 3-6 months after initiation of oral vitamin D administration and to adjust the dose, if necessary. In future, two issues have to be clarified: First, would it be more appropriate to define instead of a fixed UL a variable UL, based on the individual's body weight? Second, what are the underlying mechanisms, if any, for potentially harmful vitamin D effects at circulating 25(OH)D levels between 125 and 375 nmol/l.