Vitamin D supplements are safe for the most part when taken orally in the recommended dosage, as claimed by MedlinePlus. However, Vitamin D can cause adverse reactions in people with certain conditions and if taken in higher doses than recommended.
A person who takes too much Vitamin D may experience tiredness, headaches, weakness, decrease in appetite and stomach problems, says MedlinePlus. In some cases, doses of Vitamin D over 4,000 IU may be prescribed for a short period of time, but a dose this large should only be taken under a doctor's care. People suffering from kidney disease, high blood-calcium levels, lymphoma or tuberculosis should use Vitamin D with caution, as it may exacerbate their conditions.