Oral intake and topical administration of vitamin D, particularly vitamin D3, is likely safe in daily doses up to 4,000 IU a day, states Mayo Clinic. Most people will not experience adverse side effects from a 2,000 IU dose.
Excessive intake of vitamin D in amounts higher than the recommended dosage may result in loss of strength, exhaustion, drowsiness, headache, appetite loss, dizziness, retching and dry mouth. Chronic use of vitamin D that exceeds 4,000 IU a day may also lead to elevated levels of calcium in the blood, notes WebMD.
Oral and intramuscular administration of 900,000 IU of vitamin D on an annual basis is possibly safe for treating vitamin D deficiency. This amount is roughly equivalent to 2,500 IU of vitamin D per day.