Vitamin D is commonly available in two different forms; D-2 (ergocalciferol) and D-3 (cholecalciferol). Either one can be used to treat vitamin D deficiency although there is some evidence that
vitamin D-3 is more potent and more effective than vitamin D-2 in reversing vitamin D deficiency.
When someone is deficient physicians will often prescribe 50,000 International Units of vitamin D-2 to be given weekly for 8-12 weeks. That dosage is only available by prescription. After that initial therapy which should increase the vitamin D level to greater than 30 ng/ml the dose is typically reduced to a daily dose ranging from 1,000 to 2,000 International Units per day which can be purchased without a prescription. The Institute of Medicine states that the daily vitamin D dose for adults should not exceed 4,000 International Units per day.