Vitamin D3 is important for calcium regulation and deficiency can lead to rickets in children and osteoporosis in adults. Vitamin D3 supplements can treat or prevent osteoporosis, rickets, cardiovascular disease, psoriasis, diabetes, and have therapeutic uses for hyperparathyroidism, hypoparathyroidism, and inflammatory bowel disease. High doses of Vitamin D3 supplements can cause side effects like hypercalcemia, renal impairment, nausea, and vomiting. Recommended daily doses vary from 400 IU for infants to 1000 IU for adults over 70, but higher doses may be used to treat conditions like resistant rickets or hypoparathyroidism under medical supervision.