A daily intake of 4000 IU of vitamin D3 is generally considered the upper tolerable intake level for most adults, meaning it is the highest daily dose likely to pose no risk of adverse effects for the general population. However, supplementation at this level may increase the risk of hypercalcemia (high blood calcium) and some other adverse events in a small proportion of individuals. Hypercalcemia occurred in about 4 cases per 1000 individuals taking 3200–4000 IU daily in long-term studies. Additionally, risks of falls and hospitalization were slightly increased in those taking these doses. Vitamin D toxicity is typically associated with much higher doses (above 10,000 IU daily), but even doses at or below 4000 IU may cause issues in sensitive individuals or over long periods. For this reason, 4000 IU daily should not be exceeded without medical supervision. In summary, 4000 IU vitamin D3 daily is mostly safe for adults but carries a small increased risk of adverse effects and should be taken under guidance especially if used long- term or combined with other supplements or medications.
