What it is
The naturally occurring form of vitamin D produced in skin upon UVB exposure. Available as oral capsules, tablets, and liquid drops.
How it works
Converted in the liver to 25-hydroxyvitamin D and in the kidney to 1,25-dihydroxyvitamin D (calcitriol), the active hormone, which binds the vitamin D receptor to regulate calcium homeostasis and gene expression.
Where it's used
Used for vitamin D deficiency, bone health, and as a general dietary supplement.
FDA-approved use
Regulated as a dietary supplement at typical strengths. Prescription forms are used for documented deficiency states.
Tracking it
Vitamin D3 is oral, which makes consistency the whole game — a simple daily check-off with a reminder beats memory every time. with a half-life of about 15 days (360 h), a dose log also lets a tracker model the relative amount still in your system between doses.
Source
Public reference
Last reviewed 2026-06-11