Vitamin D3 (cholecalciferol) is a fat-soluble vitamin. The liver turns it into 25-hydroxyvitamin D (25(OH)D, the form measured in a vitamin D blood test), and the kidney turns that into the active hormone 1,25-dihydroxyvitamin D.
Also known as: cholecalciferol.
For vitamin D, 1 mcg is 40 IU. The NIH Office of Dietary Supplements lists a Recommended Dietary Allowance of 15 mcg (600 IU) a day for ages 1 to 70, including pregnancy and breastfeeding, and 20 mcg (800 IU) over 70. The Tolerable Upper Intake Level for ages 9 and up is 100 mcg (4,000 IU) a day.
It is sold in the US as a dietary supplement. Under the 1994 dietary supplement law (DSHEA), FDA does not approve dietary supplements before they are marketed, and the maker, not FDA, is responsible for safety and labeling. FDA-approved vitamin D drugs are a different matter: ergocalciferol (vitamin D2) is a prescription capsule, and cholecalciferol is part of the combination osteoporosis tablet Fosamax Plus D.
The NIH Office of Dietary Supplements (ODS) says vitamin D toxicity almost always comes from too much vitamin D from supplements, and causes high blood calcium. It lists orlistat, statins, steroids and thiazide diuretics among medicines that interact with vitamin D, and notes that fat in the meal helps absorption. A 25(OH)D blood test is the usual way to check vitamin D status; what level to aim for is a question for your clinician.
Vitamin D has more than one half-life. ODS and a review by Jones (2008) give about 15 days for 25(OH)D, the circulating form; the active hormone lasts hours, and whole-body stores turn over in about 2 months. After a single large dose of D3, 25(OH)D kept rising for about 14 days (Armas 2004; Mentaverri 2019). Dose Track models 25(OH)D, with a 15-day half-life and a peak at 14 days.
Numbers shown are the values Dose Track uses, listed only where they agree with the source above. Half-life varies from person to person and with injection site.