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Vitamin C

Evidence Fact Sheet

Vitamin C (ascorbic acid) is an essential water-soluble antioxidant and enzyme cofactor for collagen and catecholamine synthesis and non-heme iron absorption. Research dose ranges from RDA-level (75-90 mg/day) to supplemental 100-1000 mg/day. Evidence shows modest cold-duration effects but notable honest negatives in CVD prevention and ICU sepsis.

Also known as: Ascorbic Acid · L-Ascorbic Acid · Ascorbate · Vitamina C · Acido ascorbico · Vitamin C (as ascorbic acid) · Sodium ascorbate · Calcium ascorbate

Overview

Vitamin C (ascorbic acid; also sodium/calcium ascorbate) is an essential water-soluble vitamin that acts as an antioxidant scavenging reactive oxygen species, an obligatory cofactor for prolyl/lysyl hydroxylases in collagen biosynthesis and for dopamine beta-hydroxylase in catecholamine synthesis, and a reducing agent that converts dietary non-heme iron to its more absorbable ferrous form. Reference intakes are RDA 90 mg/day (men) and 75 mg/day (women) in the US and a PRI of 110/95 mg/day (EFSA), with common supplemental doses of 100-1000 mg/day and a Tolerable Upper Intake Level of 2000 mg/day. It is GRAS in the US (structure/function statements only; no SSA disease claim), carries authorized EFSA Article 13(1) function claims under Reg 432/2012, and is an authorized/permitted nutrient under ANVISA (Brazil) and China's health-food framework. This is an evidence-reporting overview, not clinical or dosing guidance.

Mechanism of Action

water-soluble antioxidant scavenging reactive oxygen species (ROS) · essential cofactor for prolyl and lysyl hydroxylase enzymes in collagen biosynthesis · cofactor for dopamine beta-hydroxylase in catecholamine (norepinephrine) synthesis · reduces dietary non-heme iron from ferric (Fe3+) to ferrous (Fe2+) form enhancing intestinal absorption · regenerates oxidized alpha-tocopherol (vitamin E) back to its reduced active form · cofactor for carnitine biosynthesis supporting fatty acid transport into mitochondria for energy production · modulates immune cell function including neutrophil chemotaxis, phagocytosis and lymphocyte proliferation

Body systems: METABOLISM

Evidence-Based Benefits

Each benefit below is anchored to a specific PubMed-indexed study. Effect sizes, sample sizes, and p-values are reported as published; no values are inferred. Honest negatives and null results are kept alongside the positive findings, and disease-research populations are described as such — Vitamin C is not characterized as a treatment for any disease.

Common Cold — Severity & Duration

Meta-analysis supported
  • 15% (95% CI 9-21%)severity reduction
  • 10 trials≥1 g/day, double-blind
  • P = 0.002severe vs mild symptoms

A meta-analysis of double-blind trials using at least 1 g/day found vitamin C modestly reduced common-cold severity and shortened the duration of severe symptoms, while having no significant effect on the duration of mild symptoms. This is a research finding on symptom course, not a cure.

Reported effect: Vitamin C significantly decreased the severity of the common cold by 15% (95% CI 9-21%); significant benefit on duration of severe symptoms but no significant effect on duration of mild symptoms (P = 0.002).

“Vitamin C significantly decreased the severity of the common cold by 15% (95% CI 9-21%). ... A subset of 5 comparisons showed vitamin C had a significant benefit on the duration of severe symptoms ... vitamin C had no significant effect on the duration of mild symptoms ... P = 0.002. The analysis examined 15 comparisons from 10 double-blind trials using vitamin C doses of at least 1 g/day.”

Source: PMID 38082300 · Hemilä 2023 · BMC Public Health

Common Cold — Prevention (General Population)

Null / no benefit Meta-analysis supported
  • RR 0.97incidence, general pop.
  • 29 trials · 11,306regular supplementation
  • RR 0.48athletes/soldiers subgroup

In this Cochrane review, regular vitamin C supplementation did NOT reduce common-cold incidence in the general population (pooled RR 0.97), so routine use is not justified; an exception was a subgroup of people under heavy physical stress (athletes, soldiers; RR 0.48). An honest negative on prevention.

Reported effect: Regular supplementation did not reduce cold incidence in the general population (pooled RR 0.97 across 29 trials, 11,306 participants); subgroup of athletes/soldiers RR 0.48; duration reduced 8% in adults and 14% in children.

“The review analyzed 29 trials with 11,306 participants examining regular vitamin C supplementation. In the general population, the pooled risk ratio was 0.97, indicating routine vitamin C supplementation is not justified for preventing colds. However, five trials of athletes and soldiers showed a pooled risk ratio of 0.48 ... regular supplementation reduced [duration] by 8% in adults and 14% in children.”

Source: PMID 23440782 · Hemilä 2013 · Cochrane Database Syst Rev

Cardiovascular Disease — Primary Prevention

Null / no benefit Meta-analysis supported
  • HR 0.99 (95% CI 0.89-1.10)major CV events
  • 8 RCTs · 15,445participants

A Cochrane systematic review found no evidence that vitamin C supplementation reduces cardiovascular disease risk in healthy people or those at increased risk; the largest trial showed no difference in major cardiovascular events over 8 years. Evidence quality was rated low to very low. An honest negative.

Reported effect: No significant difference vs placebo for major cardiovascular events (HR 0.99, 95% CI 0.89-1.10) in the largest trial (14,641 participants); 8 RCTs, 15,445 total participants.

“The largest trial (14,641 participants) found no significant difference between vitamin C and placebo groups for major cardiovascular events over 8 years (HR 0.99, 95% CI 0.89-1.10). ... there is no evidence to suggest that vitamin C supplementation reduces the risk of CVD in healthy participants and those at increased risk of CVD. ... 8 randomized controlled trials with 15,445 total participants.”

Source: PMID 28301692 · Al-Khudairy 2017 · Cochrane Database Syst Rev

Blood Pressure (Essential Hypertension)

Null / no benefit Meta-analysis supported
  • 23 studies · 2218network meta-analysis
  • no evidencevitamin C on DBP/24h BP/HR

In a network meta-analysis of vitamins for essential hypertension, only vitamin E (not vitamin C) was significantly more effective than placebo at reducing systolic blood pressure; no evidence was found that vitamin C influenced diastolic, 24-hour, or heart-rate outcomes. An honest negative for vitamin C specifically.

Reported effect: Only vitamin E significantly reduced SBP (mean difference -14.14 mm Hg, 95% credible interval -27.62 to -0.88); for the five vitamins examined, no evidence was found of an effect on DBP, 24-hour SBP/DBP, or HR; 23 studies, 2218 participants.

“only vitamin E was significantly more effective at reducing SBP with a mean difference of -14.14 mm Hg, 95% credible intervals: -27.62 to -0.88 compared to placebo. ... no evidence was found that any of the five vitamins influenced DBP, 24 hours SBP, 24 hours DBP, or HR. ... 23 studies involving 2218 participants.”

Source: PMID 38296289 · Qi 2024 · BMJ Open

Inflammation — C-Reactive Protein

RCT supported
  • 25.3% vs placebo (p=0.02)median CRP reduction
  • 1000 mg/day · 2 monthsdose · duration
  • N=396healthy nonsmokers

In a randomized trial of healthy nonsmokers, 1000 mg/day vitamin C for 2 months reduced the inflammatory marker C-reactive protein among participants with elevated baseline CRP, whereas vitamin E did not. A research finding on a biomarker, not a disease-treatment claim.

Reported effect: Among participants with elevated CRP, vitamin C reduced median CRP by 25.3% vs placebo (p=0.02; median reduction 0.25 mg/L, 16.7%); N=396 randomized to 1000 mg/day vitamin C, 800 IU/day vitamin E, or placebo for 2 months.

“Healthy nonsmokers (N=396) were randomized to three groups, 1000 mg/day vitamin C, 800 IU/day vitamin E, or placebo, for 2 months. ... vitamin C reduced the median CRP by 25.3% vs placebo (p=0.02) with median reduction in the vitamin C group, 0.25 mg/L, 16.7%.”

Source: PMID 18952164 · Block 2009 · Free Radic Biol Med

Iron-Deficiency Anemia — Vitamin C as Iron Co-Supplement

Null / no benefit RCT supported
  • 0.16 g/dL (95% CI -0.03 to 0.35)Hb difference at 2 wk
  • 440 adultsequivalence trial

In a randomized equivalence trial, adding 200 mg vitamin C to oral iron produced essentially the same hemoglobin recovery as iron alone (2.00 vs 1.84 g/dL at 2 weeks), supporting that vitamin C supplements are not essential when taking oral iron for iron-deficiency anemia. An honest negative against the common co-supplementation assumption.

Reported effect: Hemoglobin at 2 weeks rose 2.00 g/dL (iron + vitamin C) vs 1.84 g/dL (iron alone); between-group difference 0.16 g/dL (95% CI -0.03 to 0.35); 440 adults, oral iron 100 mg plus vitamin C 200 mg vs iron alone every 8 h for 3 months.

“Iron + vitamin C group: 2.00 g/dL increase; Iron-only group: 1.84 g/dL increase; Between-group difference: 0.16 g/dL (95% CI, -0.03 to 0.35). ... oral iron supplements alone were equivalent to oral iron supplements plus vitamin C ... 440 adults with iron deficiency anemia.”

Source: PMID 33136134 · Li 2020 · JAMA Netw Open

Collagen Synthesis (Exercise/Connective Tissue)

RCT supported
  • n=8crossover RCT

In a small randomized crossover trial, vitamin C-enriched gelatin taken before intermittent exercise doubled blood levels of a collagen-synthesis marker (amino-terminal propeptide of collagen I), with serum from supplemented subjects improving engineered-ligament collagen content. A mechanistic biomarker finding in healthy volunteers.

Reported effect: Subjects who took 15 g gelatin 1 h before exercise showed double the amino-terminal propeptide of collagen I (PINP) in blood, indicating increased collagen synthesis; 8 healthy male subjects, randomized double-blind crossover, 5 or 15 g vitamin C-enriched gelatin vs placebo.

“Subjects who took 15 g gelatin 1 h before exercise showed double the amino-terminal propeptide of collagen I in their blood, indicating increased collagen synthesis. ... Eight healthy male subjects in a randomized, double-blinded, crossover design ... consumed either 5 or 15 g of vitamin C-enriched gelatin or a placebo control.”

Source: PMID 27852613 · Shaw 2017 · Am J Clin Nutr

Bioavailability — Liposomal vs Standard Delivery

RCT supported
  • 27% higher peak · 21% AUCplasma · p<0.001
  • n=27500 mg dose

A double-blind randomized trial found that liposomal vitamin C increased absorption into both plasma and leukocytes compared with standard vitamin C at an equivalent 500 mg dose. A pharmacokinetic finding on delivery form, not a clinical outcome.

Reported effect: Liposomal formulation showed 27% higher plasma peak and 21% greater plasma AUC, and 20% higher leukocyte peak with 8% greater leukocyte AUC (all p < 0.001); n = 27 (19 males, 8 females), 500 mg equivalent doses.

“Plasma concentration: Liposomal formulation showed 27% higher peak levels and 21% greater area under the curve; Leukocyte concentration: 20% higher peak levels and 8% greater area under the curve; All comparisons achieved p < 0.001. ... 19 males and 8 females (n = 27; 36.0 ± 5.1 years).”

Source: PMID 39237620 · Purpura 2024 · Eur J Nutr

Sepsis / Critical Illness (IV Vitamin C)

Null / no benefit RCT supported
  • RR 1.21 (95% CI 1.04-1.40)death/organ dysfunction d28
  • P = 0.01 · n=863LOVIT trial

In the large LOVIT randomized trial, intravenous vitamin C in septic adults on vasopressors INCREASED the risk of death or persistent organ dysfunction at day 28 versus placebo. This is a high-profile honest negative (signal of harm) that overturned earlier promising preliminary findings; an earlier trial (CITRIS-ALI) had already shown no benefit on its primary organ-dysfunction outcome.

Reported effect: Death or persistent organ dysfunction at day 28 occurred in 44.5% (191/429) of vitamin C recipients vs 38.5% (167/434) controls; risk ratio 1.21 (95% CI 1.04-1.40; P = 0.01).

“The primary composite outcome of death or persistent organ dysfunction ... on day 28 occurred in 44.5% (191/429) of vitamin C recipients versus 38.5% (167/434) controls, yielding a risk ratio, 1.21; 95% confidence interval [CI], 1.04 to 1.40; P = 0.01. ... had a higher risk of death or persistent organ dysfunction at 28 days than those who received placebo.”

Source: PMID 35704292 · Lamontagne 2022 · N Engl J Med

Antioxidant Support — Endometriosis-Associated Pain

Meta-analysis supported
  • RR 7.30 (95% CI 3.27-16.31)chronic pelvic pain, p<0.00001
  • 5 RCTsvitamin C + E

A meta-analysis of five randomized trials reported that combined vitamin C and E antioxidant supplementation was associated with a higher proportion of women with endometriosis reporting reduced chronic pelvic pain, dysmenorrhea, and dyspareunia. A research finding in a studied population; vitamin C was given combined with vitamin E.

Reported effect: Higher proportion reporting reduced chronic pelvic pain (RR 7.30; 95% CI 3.27-16.31, p<0.00001, I2=0%); dysmenorrhea (RR 1.96; 95% CI 1.25-3.07, p=0.003); dyspareunia (RR 5.08; 95% CI 2.10-12.26, p=0.0003); 5 RCTs.

“higher proportion of endometriosis patients reporting reduced chronic pelvic pain (RR 7.30; 95%CI: 3.27-16.31, p<0.00001, I2 = 0%) ... alleviations of dysmenorrhea (RR 1.96; 95%CI: 1.25-3.07, p = 0.003, I2 = 39%) ... dyspareunia (RR 5.08; 95%CI: 2.10-12.26, p = 0.0003, I2 = 0%). The meta-analysis of five randomized controlled trials ...”

Source: PMID 38820340 · Bayu 2024 · PLoS One

Cancer Risk — Genetic (Mendelian Randomization)

Null / no benefit Meta-analysis supported
  • OR 1.05 (95% CI 0.94-1.17)breast, replication cohort

A two-sample Mendelian randomization study found no evidence that genetically predicted circulating vitamin C concentration is causally associated with lung, breast, prostate, colon, or rectal cancer; an initial breast-cancer signal failed to replicate in a larger consortium. An honest negative on causal cancer prevention.

Reported effect: No evidence of a causal association of circulating vitamin C with any examined cancer; UK Biobank breast-cancer OR 1.34 (95% CI 1.14-1.57) per SD did not replicate (OR 1.05, 95% CI 0.94-1.17).

“The MR analysis showed no evidence of a causal association of circulating vitamin C concentration with any examined cancer. ... In UK Biobank, genetically predicted vitamin C showed OR 1.34 (95% CI 1.14-1.57) per SD increase, but this failed replication in the Breast Cancer Association Consortium (OR 1.05, 95% CI 0.94-1.17).”

Source: PMID 34325683 · Fu 2021 · BMC Med

Dosage (research context · not a recommendation)

EFSA NRV (Nutrient Reference Value, used for labelling): 80 mg/day. EFSA Population Reference Intake (PRI): 110 mg/day adult men, 95 mg/day adult women (increased by 10 mg/day during pregnancy and 60 mg/day during lactation). US IOM RDA: 90 mg/day adult men, 75 mg/day adult women (+35 mg/day for smokers). Common supplemental doses: 100-1000 mg/day. EFSA-conditioned immune-during-exercise claim requires daily intake of 200 mg vitamin C. US IOM Tolerable Upper Intake Level (UL): 2000 mg/day for adults (based on osmotic diarrhea risk).

Regulatory Status · 4 Markets

US · FDA
United States: Vitamin C (ascorbic acid) is recognized as an essential nutrient with an established Daily Value (90 mg) under 21 CFR 101.9. There is NO authorized Significant Scientific Agreement (SSA) health claim for vitamin C under 21 CFR Part 101 Subpart E (SSA health claims are limited to twelve specified substances such as calcium, sodium, dietary fat, saturated fat, dietary fiber, fruits and vegetables, folate, sugar alcohols, soluble fiber, soy protein and plant sterols/stanols; vitamin C is not among them). Permitted communications are limited to: (a) nutrient content claims (e.g. "Excellent source of vitamin C" if ≥20% DV per serving, "Good source" if 10-19% DV per serving) under 21 CFR 101.54; and (b) DSHEA structure/function statements (e.g. "supports immune function", "supports antioxidant defenses") under 21 USC 343(r)(6), which must bear the FDA disclaimer ("This statement has not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.") and be substantiated, truthful and not misleading. Qualified Health Claims (QHCs) and disease prevention/treatment claims are NOT permitted for vitamin C in the absence of a specific FDA letter of enforcement discretion.
EU · EFSA
European Union: Vitamin C is included on the EU permitted list of vitamins for food supplements (Directive 2002/46/EC) and in fortified foods (Regulation (EC) No 1925/2006). The Nutrient Reference Value (NRV) for labelling is 80 mg (Regulation (EU) No 1169/2011 Annex XIII). Fifteen Article 13(1) function health claims for vitamin C are authorized under Commission Regulation (EU) No 432/2012 (Annex), all conditioned on the food providing at least a "source" of vitamin C (≥15% NRV per 100 g/100 ml/portion) under Regulation (EC) No 1924/2006, with the exception of the claim "Vitamin C contributes to maintain the normal function of the immune system during and after intense physical exercise" which requires a daily intake of 200 mg vitamin C. EFSA Panel on Dietetic Products, Nutrition and Allergies set the Population Reference Intake at 110 mg/day for adult men and 95 mg/day for adult women (2013 opinion).
CN · China
Listed in the Health-Food Raw-Material Catalogue (Nutrient Supplements) 2020 edition under filing/notification route, and a permitted food nutrition fortifier (GB 14880-2012) and food additive (GB 2760). Notified supplements may claim 'supplements vitamin C'; registered health foods may bear approved function claims such as immune enhancement / antioxidant. Ordinary foods bearing vitamin C may not make any health-function claim. SAMR recognized.
BR · ANVISA
Brasil: A vitamina C (acido ascorbico) e reconhecida como nutriente essencial pela ANVISA. A Instrucao Normativa IN 28/2018 (DC/ANVISA) estabelece os constituintes autorizados, limites minimos e maximos por dose diaria, alegacoes funcionais (Anexo V) e rotulagem complementar para suplementos alimentares. A IDR (Ingestao Diaria Recomendada) de vitamina C para adultos no Brasil e de 45 mg/dia (RDC 269/2005), com Limite Maximo de Seguranca para suplementacao definido em IN 28/2018 Anexo III. As alegacoes funcionais autorizadas no Anexo V da IN 28/2018 seguem o padrao verbatim oficial "A vitamina C auxilia..." e cobrem funcionamento do sistema imune, formacao do colageno, metabolismo energetico, antioxidante (protecao das celulas contra os radicais livres) e regeneracao da vitamina E.

Authorized Claims

ANVISA — “A vitamina C auxilia no funcionamento do sistema imune.” (ANVISA IN 28/2018 Anexo V)

Safety

Generally recognized as safe at typical supplemental doses. Doses above the UL of 2000 mg/day may cause gastrointestinal disturbances including osmotic diarrhea, nausea and abdominal cramps. High-dose vitamin C may increase oxalate excretion and theoretically increase risk of calcium oxalate kidney stones in predisposed individuals. Patients with hereditary hemochromatosis, glucose-6-phosphate dehydrogenase (G6PD) deficiency or iron overload conditions should consult a healthcare professional before taking high-dose supplements due to enhanced iron absorption and potential pro-oxidant effects. May interfere with certain laboratory tests (glucose, occult blood). Pregnancy and lactation: do not exceed UL.

Goals: inflammation-relief

Lifestyles: senior-60-plus

References

PubMed-indexed citations anchoring the benefit findings above. Effect sizes are reported as published.

  1. PMID 38082300 · Hemilä 2023 · BMC Public Health — Common Cold — Severity & Duration
  2. PMID 23440782 · Hemilä 2013 · Cochrane Database Syst Rev — Common Cold — Prevention (General Population)
  3. PMID 28301692 · Al-Khudairy 2017 · Cochrane Database Syst Rev — Cardiovascular Disease — Primary Prevention
  4. PMID 38296289 · Qi 2024 · BMJ Open — Blood Pressure (Essential Hypertension)
  5. PMID 18952164 · Block 2009 · Free Radic Biol Med — Inflammation — C-Reactive Protein
  6. PMID 33136134 · Li 2020 · JAMA Netw Open — Iron-Deficiency Anemia — Vitamin C as Iron Co-Supplement
  7. PMID 27852613 · Shaw 2017 · Am J Clin Nutr — Collagen Synthesis (Exercise/Connective Tissue)
  8. PMID 39237620 · Purpura 2024 · Eur J Nutr — Bioavailability — Liposomal vs Standard Delivery
  9. PMID 35704292 · Lamontagne 2022 · N Engl J Med — Sepsis / Critical Illness (IV Vitamin C)
  10. PMID 38820340 · Bayu 2024 · PLoS One — Antioxidant Support — Endometriosis-Associated Pain
  11. PMID 34325683 · Fu 2021 · BMC Med — Cancer Risk — Genetic (Mendelian Randomization)

Frequently Asked Questions

1. Does vitamin C prevent the common cold?

In the general population, no. A Cochrane meta-analysis of 29 trials (11,306 participants) found regular supplementation did not reduce cold incidence (pooled RR 0.97), so routine use was deemed unjustified for prevention. An exception appeared in a subgroup of people under heavy physical stress, such as athletes and soldiers (RR 0.48). This is a research finding, not medical advice.

2. If it doesn't prevent colds, does vitamin C do anything for them?

The evidence points to a modest effect on the course of symptoms rather than prevention. A 2023 meta-analysis of 10 double-blind trials using at least 1 g/day found vitamin C reduced cold severity by 15% (95% CI 9-21%) and shortened the duration of severe symptoms, but had no significant effect on the duration of mild symptoms. The Cochrane review similarly reported duration reductions of 8% in adults and 14% in children with regular use.

3. Should I take vitamin C with my iron supplement?

A randomized equivalence trial in 440 adults with iron-deficiency anemia found adding 200 mg vitamin C to oral iron gave essentially the same hemoglobin recovery as iron alone (a between-group difference of 0.16 g/dL, 95% CI -0.03 to 0.35), leading the authors to conclude vitamin C supplements are not essential alongside oral iron. Vitamin C does chemically reduce non-heme iron to a more absorbable form, but in this trial it added no measurable benefit. This is evidence reporting, not dosing guidance.

4. What about high-dose or intravenous vitamin C in serious illness?

The strongest signal here is a caution. In the large LOVIT randomized trial in N Engl J Med, intravenous vitamin C in septic adults on vasopressors increased the risk of death or persistent organ dysfunction at day 28 (44.5% vs 38.5%; risk ratio 1.21, 95% CI 1.04-1.40, P = 0.01) compared with placebo. Earlier work (CITRIS-ALI) had already shown no benefit on its primary organ-dysfunction outcome. These are hospital/ICU research contexts, not self-supplementation scenarios.

5. Is liposomal vitamin C better absorbed than regular vitamin C?

A double-blind randomized trial (n=27) at an equivalent 500 mg dose found liposomal vitamin C reached 27% higher plasma peak levels and 21% greater plasma AUC, plus higher leukocyte uptake, versus standard vitamin C (all p < 0.001). That demonstrates a pharmacokinetic advantage in absorption; it does not, by itself, establish a difference in any clinical health outcome.

6. Does vitamin C lower inflammation, blood pressure, or cardiovascular and cancer risk?

The findings are mixed and area-specific. A randomized trial found 1000 mg/day vitamin C reduced the inflammatory marker CRP by 25.3% versus placebo (p=0.02) in people with elevated baseline CRP. However, a Cochrane review of 8 RCTs (15,445 participants) found no evidence vitamin C prevents cardiovascular disease (largest trial HR 0.99, 95% CI 0.89-1.10), a network meta-analysis found no evidence it affects blood pressure, and a Mendelian randomization study found no causal link to lung, breast, prostate, colon, or rectal cancer. All of these are research findings in studied populations, not treatment claims.

Last evidence review: 2026-06-04

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