2000 IU is an appropriate daily dose of vitamin D3 for adults, according to WebMD. The recommended daily dose for all adults above the age of 18, including women who are pregnant or nursing, is between 1,500 and 2,000 IU.
Vitamin D3 should not be taken in doses exceeding the recommended daily amount except in cases of severe vitamin D deficiency, notes WebMD. The side effects of excessive vitamin D3 use include headaches, nausea, tiredness, drowsiness and decreased appetite. Extremely high doses of vitamin D3 can also lead to elevated calcium levels, which can result in atherosclerosis and kidney stones.