Vitamin C: Benefits, Dosage & Side Effects (2026)

Vitamin C (L-ascorbic acid, or ascorbate) is a water-soluble essential vitamin. Unlike most animals, humans lost the gene for the final enzyme in vitamin C synthesis (L-gulonolactone oxidase), so we cannot make our own and must get it from food — citrus, peppers, broccoli, kiwi and other fruits and vegetables. Biochemically it is a cofactor for a family of enzymes (including the collagen-building hydroxylases) and a potent antioxidant that also regenerates other antioxidants like vitamin E. Frank deficiency causes scurvy; the RDA is modest — about 90 mg/day for men and 75 mg/day for women, with smokers needing roughly 35 mg more.

The single most important thing to understand before buying vitamin C is that oral absorption is tightly regulated and saturable. Plasma and tissue levels plateau at intakes around 200 mg/day; above that, fractional absorption falls and the excess is excreted in urine. Levine 1996 (PMID 8623000) mapped this depletion-repletion curve in healthy volunteers, and Padayatty 2004 (PMID 15068981) showed that at a 1.25 g oral dose bioavailability drops below 50%, while only intravenous administration can push plasma into the high pharmacologic range. In plain terms: for oral vitamin C, more is not more — past a couple hundred milligrams, you largely excrete the difference.

That reframes the whole supplement question. Unlike curcumin, where the battle is bioavailable formulation, vitamin C's active form is cheap, well-absorbed plain ascorbic acid — the buyer's real mistake is dose expectations, not form. A sensible daily immune dose sits in the 200 mg to 1 g range: enough to keep leukocytes and tissues saturated, cheap, and safe. Exotic liposomal or buffered products are fine but rarely necessary, and gram-scale "immune megadoses" mostly buy you expensive urine rather than a proportionally bigger effect.

Vitamin C's link to immunity is biologically real. It accumulates to high concentrations inside immune cells — neutrophils, lymphocytes and monocytes actively concentrate ascorbate — where it supports chemotaxis (the cells' ability to migrate to infection), phagocytosis, oxidant handling during the respiratory burst, and the apoptosis and clearance of spent neutrophils. It also underwrites the physical barrier: as a cofactor for collagen synthesis it helps maintain skin and epithelial integrity, the body's first line against pathogens. Deficiency measurably impairs these functions and raises susceptibility to infection, which is why correcting a genuine shortfall matters.

But the leap from "supports immune cells" to "take it to beat colds" is where the evidence turns honest and unglamorous. Hemilä & Chalker 2013, the Cochrane review (PMID 23440782), pooled 29 controlled trials in more than 11,000 people and found that regular prophylactic supplementation did not reduce the incidence of colds in the general population — you don't catch fewer colds. It did produce a modest, consistent reduction in cold duration (about 8% in adults and 14% in children) and severity when taken continuously beforehand. The one striking exception: in people under short bursts of extreme physical stress — marathon runners, skiers, soldiers on subarctic exercises — regular vitamin C roughly halved cold incidence. And critically, trials that started high-dose vitamin C only after symptoms appeared showed no consistent benefit.

The pharmacokinetics explain why megadosing doesn't rescue this modest picture. Because oral absorption saturates near 200 mg/day (Levine 1996, PMID 8623000) and bioavailability collapses at gram-scale doses (Padayatty 2004, PMID 15068981), swallowing 2,000 mg cannot flood immune cells with proportionally more ascorbate — the plasma ceiling holds. So the honest synthesis is: vitamin C is a true immune-cell cofactor and repairing deficiency is worthwhile, but for a well-nourished person the payoff of supplementing is a slightly shorter, milder cold — a duration effect, not a prevention effect — and only extreme-exertion athletes see incidence drop.