The upper limit of Vitamin D3 intake that is considered safe is 4,000 IU per day for adults over the age of 50. However, it is widely recommended that people do not exceed 2,000 IU per day, as consuming more than that amount can lead to toxicity. Symptoms of Vitamin D3 overdose may include nausea, vomiting, loss of appetite, weakness, frequent urination, and kidney stones.