Vitamin D (typically D3) is found in multivitamin/mineral formulations and is also sold as a single supplement or combined with calcium. Until recently most supplements contained 400 International Units (IU), which is the Daily Value used on labeling. But doses have been increasing, and now you’ll often see pills containing 1,000 to 10,000 IU. There are prescription capsules with even higher amounts for weekly dosing.
If you are tested and are found to have very low blood levels of vitamin D, your doctor may prescribe high short-term “loading” doses—such as 50,000 IU a week for six to eight weeks. Such high dosing should be done only under medical supervision. After that, a dose of 800 to 2,000 IU a day is typically used to maintain blood levels.
Though estimates vary, blood levels of vitamin D rise by about 1 ng/mL for every 100 IU of additional vitamin D consumed each day for at least several weeks. For example, someone starting with a blood level of 15 ng/mL would require, on average, about an additional 1,500 IU a day to bring it up to 30 ng/mL. Keep in mind, however, that individual responses to supplemental doses vary widely.