Hi doses of Vitamin D3 can have side effects, headaches, nausea, lethargy, and even kidney stones. That being said I personally take 5-10 thousand IU per day. My wife takes it also and has her Vit D levels checked by her Ob-gyn who has had her take it for 20 years. He is a big believer in the Vitamin D, and says many women are low on it. Her levels have always been ok.