Vitamin D deficiency is a growing global health issue, with significant implications for skeletal health, immune function, and overall mortality. Recent studies indicate that up to 40% of Europeans and 13% are severely deficient, highlighting the need for supplementation. However, many interventional trials have failed to show clear benefits due to methodological flaws, such as small sample sizes and inadequate vitamin D-replete populations. A 25(OH)D level of >50 nmol/L is recommended as the primary treatment goal, with severe deficiency (<30 nmol/L) posing serious health risks. Vitamin D is not a panacea but may be an effective and safe adjuvant therapy for various diseases. Public health interventions, including targeted supplementation and food fortification, are crucial to prevent severe deficiency. Current research suggests that vitamin D supplementation can reduce mortality, infections, and chronic diseases, particularly in severely deficient individuals. However, the optimal dose and duration of supplementation remain debated. Recent trials indicate that daily dosing is more effective than long-term intervals, with a recommended daily dose of 20–50 µg (800–2000 IU) to achieve target 25(OH)D levels. Vitamin D toxicity is rare but possible at very high doses, and safety guidelines recommend upper limits of 4000 IU/day. Vitamin D deficiency is also linked to adverse pregnancy outcomes, increased risk of diabetes, and poor outcomes in critically ill patients. Despite these benefits, the evidence for vitamin D's role in cancer prevention remains inconclusive. Future research should focus on large, well-designed trials to clarify its therapeutic potential. Public health strategies, including vitamin D fortification and targeted supplementation, are essential to address the global burden of vitamin D deficiency.Vitamin D deficiency is a growing global health issue, with significant implications for skeletal health, immune function, and overall mortality. Recent studies indicate that up to 40% of Europeans and 13% are severely deficient, highlighting the need for supplementation. However, many interventional trials have failed to show clear benefits due to methodological flaws, such as small sample sizes and inadequate vitamin D-replete populations. A 25(OH)D level of >50 nmol/L is recommended as the primary treatment goal, with severe deficiency (<30 nmol/L) posing serious health risks. Vitamin D is not a panacea but may be an effective and safe adjuvant therapy for various diseases. Public health interventions, including targeted supplementation and food fortification, are crucial to prevent severe deficiency. Current research suggests that vitamin D supplementation can reduce mortality, infections, and chronic diseases, particularly in severely deficient individuals. However, the optimal dose and duration of supplementation remain debated. Recent trials indicate that daily dosing is more effective than long-term intervals, with a recommended daily dose of 20–50 µg (800–2000 IU) to achieve target 25(OH)D levels. Vitamin D toxicity is rare but possible at very high doses, and safety guidelines recommend upper limits of 4000 IU/day. Vitamin D deficiency is also linked to adverse pregnancy outcomes, increased risk of diabetes, and poor outcomes in critically ill patients. Despite these benefits, the evidence for vitamin D's role in cancer prevention remains inconclusive. Future research should focus on large, well-designed trials to clarify its therapeutic potential. Public health strategies, including vitamin D fortification and targeted supplementation, are essential to address the global burden of vitamin D deficiency.