Vitamin D

D3 (cholecalciferol) · D2

The most-tested supplement of the last decade. Correcting a deficiency clearly matters; giving it to people who aren't deficient has now failed in several of the largest trials ever run on a supplement.

last reviewed 2026-08-30

by draft — unverified

GRADE

CLAIM

SPECIES

EFFECT

STUDIES

correcting measured deficiency — grade A, large effect (human-rct)

preventing cancer & cardiovascular disease in non-deficient adults — grade E, negligible effect (human-rct)

Manson et al. (VITAL) (2019)

species humandesign RCT, double-blindn 25871duration 5.3 years mediandose 2000 IU/dayfunding publicregistry NCT01169259outcome invasive cancer, major cardiovascular events

No effect on either primary endpoint in a generally replete population. Tested at scale, found not to work — for this claim.

fracture prevention in deficient or institutionalised elderly (with calcium) — grade B, small effect (human-rct)

Grade timeline

No grade movement since first review.

Dosing & safety

TYPICAL SUPPLEMENT DOSE1000–2000 IU/day

DOSES USED IN STUDIES400–4000 IU/day

UPPER LIMIT4000 IU/day (100 µg) for adults

SAFETY NOTE Fat-soluble and accumulates; sustained intakes above the UL can cause hypercalcemia. Blood testing is cheap — deficiency, not dose, is the question.

COST SNAPSHOT

No product records for this compound yet.

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