Vitamin C

vitamin C · ascorbic acid · ascorbate

The classic antioxidant vitamin, promoted for immunity, skin and longevity.

TypeSupplement / dietary

The grade answers: What does the human evidence support for: Antioxidant, immune & deficiency correction?

Grade

Limited

The grade rates evidence quality — it is not advice to take or buy.

Class: Water-soluble vitamin
Primary use: Antioxidant, immune & deficiency correction
Evidence strength: medium
Last reviewed: 2026-07-01

Bottom line

Essential, and correcting deficiency matters — but megadoses don't prevent colds in most people or extend life, and the body simply excretes the excess. Useful, not a super-supplement.

What the evidence says

Vitamin C is essential (humans can't synthesise it) and true deficiency (scurvy) is serious, so adequacy clearly matters. Beyond that, the evidence deflates the hype: routine supplementation doesn't prevent colds in the general population (it may slightly shorten them, and helps in extreme physical stress), large antioxidant-supplement trials haven't shown longevity benefit, and the body tightly regulates blood levels — excess is largely excreted. Topical vitamin C has a separate, better cosmetic rationale for skin. Grade C: essential nutrient, unremarkable as a megadose longevity pill.

Key studies

[1]
Vitamin C for preventing and treating the common cold · meta-analysis
No prevention in general population; minor duration effect.
Open on PubMed ↗
[2]
Antioxidant supplements and mortality · review
No longevity benefit from antioxidant megadosing.
Open on PubMed ↗
[3]
Vitamin C pharmacokinetics · review
Blood levels saturate; excess is excreted.
Open on PubMed ↗

See all studies on PubMed ↗

Scoped PubMed search, not a curated bibliography: the individual PMIDs for this entry have not yet been verified one by one. For that reason it is not used to support a high grade and is awaiting review.

Mechanism

A cofactor for collagen synthesis and several enzymes and a water-soluble antioxidant; blood levels are homeostatically capped, limiting the effect of very high oral doses.

Safety

Very safe; high oral doses mainly cause GI upset/diarrhoea and, in predisposed people, raise kidney-stone risk. Excess is excreted rather than stored.

Dosage context

Requirements are ~75–120 mg/day, easily met by diet; gram-level 'immune' doses exceed absorption and are largely wasted. Topical formulations are a different, skin-specific use.

Examples of application

Easily met from diet; supplements above absorption are largely excreted.
Gram-level 'immune' doses mostly don't prevent colds.
Topical vitamin C is a separate, skin-specific use.

From the field

Vitamin C is the original 'megadose it for everything' supplement, and the evidence mostly says your kidneys just filter out the excess. Essential in the diet, unremarkable as a longevity pill. Grade C.

Related molecules

Vitamin DC ZincC

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