NutriDex

The Supplement Research Compendium
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Vitamin C

Ascorbic acid

Classic antioxidant vitamin and collagen cofactor.

Moderate evidence 🍊Vitamin🛡️Gut & Immune

✚ Updated · How we grade evidence

Evidence tier
Moderate
Research weight
Citations
21 verified / 21
Classification
Vitamin
❝
What the evidence says. Several controlled trials; effects real but modest or context-dependent.

What is Vitamin C?

Vitamin C (Ascorbic acid) is a vitamin used for antioxidant. NutriDex grades the human evidence as Moderate. Vitamin C is a water-soluble antioxidant essential for collagen synthesis and immune function. Regular supplementation doesn't prevent colds in the general population but modestly shortens their duration. It enhances non-heme iron absorption when taken with meals. Megadoses offer little extra benefit and can cause GI upset and kidney-stone risk in susceptible people.

Purported Benefits

✓Antioxidant
✓Collagen synthesis
✓Slightly shorter colds
✓Iron absorption

Evidence by outcome

The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.

OutcomeEvidenceEffectStudies
Common cold (duration/severity)Cochrane: no prevention but ~8% shorter colds; 2023 meta cut severity 15%. Real but modest, regular dosing required. Moderate ↑ benefit · small 2
Non-heme iron absorptionMarkedly increases non-heme iron uptake when taken with meals; classic mechanistic human data. Moderate ↑ benefit · moderate 1
Blood pressureMeta-analyses (20-29 RCTs) lower systolic BP ~3-4 mmHg, more in hypertensives; short-term, clinical outcomes unproven. Moderate ↑ benefit · small 2
Glycemic control (type 2 diabetes)Meta-analysis (22 RCTs) lowered HbA1c/fasting glucose, mainly at >=1000 mg/day for >=12 wk; single pooled estimate. Preliminary ↑ benefit · small 1
IV vitamin C in sepsis/critical illnessLOVIT RCT (872 pts) increased death/organ dysfunction (RR 1.21); IV use signals harm, not benefit, in sepsis. Moderate ⚠ risk · moderate 3
Serum uric acidMeta-analyses show modest urate lowering (~-0.35 mg/dL); gout-prevention benefit unproven. Moderate ↑ benefit · small 2
All-cause/CVD mortalityCohorts link higher vitamin C to lower mortality, but this is observational; trial-based meta not confirmed by sequential analysis. Mixed ↔ mixed 3

Dosing & Compounds

Typical Dose
75–90 mg/day RDA; 200 mg saturates tissues. Doses >1g mostly excreted.
Active Compounds
L-ascorbic acid

Safety & Cautions

Safe. High doses cause diarrhea; may raise kidney-stone risk in predisposed individuals. Educational only — always check with your doctor or pharmacist before combining Vitamin C with any medicine.

Vitamin C drug interactions

Known or theoretical interactions between Vitamin C and common medications — educational, not exhaustive. Always check with your doctor or pharmacist before combining Vitamin C with any medicine.

Caution
Chemotherapy / radiation
High-dose antioxidant vitamin C may theoretically reduce some chemo/radiation efficacy — discuss with oncologist.
As an antioxidant it could neutralize reactive oxygen species that mediate cytotoxicity (controversial). NCI PDQ — High-Dose Vitamin C

Key Studies ★ 21 studies

Meta-analysis Hemilä 2023 (meta-analysis) ✓ PubMed
In 10 trials (≥1 g/day), vitamin C reduced common-cold severity by 15% (95% CI 9–21%), with a larger 26% reduction in duration of severe symptoms.
Meta-analysis Lbban 2023 (meta-analysis) ✓ PubMed
Across 20 RCTs, vitamin C supplementation lowered systolic BP by ~3.0 mmHg, with larger reductions in hypertensive (3.2 mmHg) and diabetic (4.6 mmHg) patients.
Systematic review Wang 2024 (SR/meta-analysis) ✓ PubMed
In 19 RCTs (2,047 critically ill patients), IV vitamin C reduced vasopressor and mechanical-ventilation duration but did not change 28-day mortality (RR 0.95, 95% CI 0.80–1.11).
Meta-analysis Liang 2023 (meta-analysis) ✓ PubMed
In 10 RCTs (1,426 adults with sepsis), vitamin C did not significantly reduce short-term mortality overall (OR 0.61, 95% CI 0.37–1.01), with benefit only in a developing-country subgroup.
Systematic review Overall survival IVC 2025 (SR/meta-analysis) ✓ PubMed
Across 8 studies (2,722 cancer patients), high-dose intravenous vitamin C was associated with ~83% longer median overall survival (ratio 1.83, 95% CI 1.40–2.40; moderate certainty).
systematic review / meta-analysis Nosratabadi 2023 (SR/meta-analysis) ✓ PubMed
Across 22 RCTs (1,447 adults with type 2 diabetes), vitamin C supplementation significantly lowered HbA1c, fasting blood glucose, and fasting insulin, with greatest benefit at high doses (>=1000 mg/day) for >=12 weeks.
systematic review / meta-analysis of RCTs Kow 2023 (meta-analysis) ✓ PubMed
Pooling 11 RCTs of COVID-19 patients, vitamin C administration was associated with a significant reduction in all-cause mortality (pooled OR 0.53, 95% CI 0.30-0.92).
Meta-analysis COVID-19 vitamin C 2024 ✓ Source
Systematic review/meta-analysis of RCTs comparing vitamin C with standard care in adult COVID-19 patients found no significant reduction in in-hospital mortality.
Meta-analysis Vitamin C & mortality 2023 ✓ Full text
Meta-analysis of 44 trials (26,540 participants) found oral/IV vitamin C associated with a 13% reduction in all-cause mortality, though not confirmed by trial sequential analysis.
Meta-analysis Liu 2021 (meta-analysis) ✓ PubMed
Pooled RCT data show oral vitamin C significantly lowers serum uric acid, though magnitude is modest and gout-prevention benefit is unproven.
umbrella review of systematic reviews and meta-analyses Jiang 2022 (umbrella review) ✓ PubMed
Umbrella review of 76 meta-analyses (63 outcomes) found dose-response associations of higher vitamin C intake (per 50-100 mg/day) with lower all-cause mortality and cardiovascular disease, but flagged increased risk of kidney stones and breast cancer with supplement use.
Cochrane review Hemilä 2013 (Cochrane) ✓ PubMed
Didn't prevent colds but shortened duration ~8% (adults).
Meta-analysis Juraschek (blood pressure meta-analysis) 2012 ✓ PubMed
Meta-analysis of 29 RCTs (median dose 500 mg/d, ~8 weeks): vitamin C reduced systolic BP by -3.84 mmHg and diastolic BP by -1.48 mmHg overall; in hypertensive subjects systolic reduction was -4.85 mmHg. Short-term effect; long-term/clinical outcomes unproven.
Meta-analysis Juraschek (serum uric acid meta-analysis) 2011 ✓ PubMed
Meta-analysis of 13 RCTs (median 500 mg/d, ~30 days): vitamin C significantly lowered serum uric acid by -0.35 mg/dL (95% CI -0.66 to -0.03). Modest urate-lowering effect; impact on gout incidence not established.
RCT LOVIT-COVID / REMAP-CAP (Adhikari/Lamontagne) 2023 ✓ PubMed
Two harmonized RCTs in 2590 hospitalized COVID-19 patients: IV vitamin C had low probability of benefit on organ support-free days and survival, with high posterior probability of harm in both critically ill (proportional OR 0.88) and non-critically ill (OR 0.80) strata; trials stopped for harm/futility.
RCT Lamontagne 2022 (LOVIT) ✓ Source
In 872 septic ICU adults on vasopressors, high-dose IV vitamin C (50 mg/kg q6h) increased the risk of death or persistent organ dysfunction at 28 days vs placebo (RR 1.21, 95% CI 1.04-1.40).
RCT Lamontagne (LOVIT trial) 2022 ✓ PubMed
In 872 ICU adults with sepsis on vasopressors, 4-day high-dose IV vitamin C INCREASED the risk of death or persistent organ dysfunction at 28 days vs placebo (44.5% vs 38.5%; RR 1.21, 95% CI 1.04-1.40); 28-day mortality 35.4% vs 31.6%. Signals harm, not benefit.
Observational Dietary/circulating vitamin C & CVD 2023 ✓ Full text
Dose-response meta-analysis of prospective cohorts found higher dietary intake and circulating concentrations of vitamin C each associated with lower total cardiovascular mortality risk.
Cohort Khaw (EPIC-Norfolk cohort) 2001 ✓ PubMed
Prospective study of 19,496 adults: plasma ascorbic acid was inversely associated with all-cause, cardiovascular, and ischemic heart disease mortality; top vs bottom quintile had ~half the mortality risk, and each 20 micromol/L rise was associated with ~20% lower all-cause mortality, independent of major confounders. Association, not causation.
Study Lynch 1986 ✓ PubMed
Markedly increased non-heme iron absorption.
Preclinical Carr 2017 review ✓ PubMed
Essential for immune cell function; deficiency impairs immunity.
21 sources cited · 21 verified Search all studies on PubMed ↗

Common questions about Vitamin C

What is Vitamin C used for?

Vitamin C is most often taken for Antioxidant, Collagen synthesis, Slightly shorter colds, Iron absorption. Classic antioxidant vitamin and collagen cofactor.

Does Vitamin C work — what does the evidence say?

Moderate evidence. Several controlled trials; effects real but modest or context-dependent. Vitamin C is a water-soluble antioxidant essential for collagen synthesis and immune function. Regular supplementation doesn't prevent colds in the general population but modestly shortens their duration. It enhances non-heme iron absorption when taken with meals. Megadoses offer little extra benefit and can cause GI upset and kidney-stone risk in susceptible people.

What is the typical dose of Vitamin C?

75–90 mg/day RDA; 200 mg saturates tissues. Doses >1g mostly excreted.

Is Vitamin C safe? Any cautions or side effects?

Safe. High doses cause diarrhea; may raise kidney-stone risk in predisposed individuals.

How many studies support Vitamin C?

NutriDex cites 21 sources for Vitamin C, graded "Moderate".

Does Vitamin C interact with any medications?

Yes — known or theoretical interactions include: Chemotherapy / radiation (caution). This is educational and not exhaustive; always check with your doctor or pharmacist before combining Vitamin C with any medicine.

Cite this page
APA

Peh, D. (2026). Vitamin C (Ascorbic acid): Benefits, Dosage, Side Effects & Evidence. NutriDex — The Supplement Research Compendium. Retrieved 26 Jun 2026, from https://nutridex.info/s/vitaminc

BibTeX
@misc{nutridex_vitaminc,
  author       = {Peh, Daryl},
  title        = {Vitamin C (Ascorbic acid): Benefits, Dosage, Side Effects \& Evidence},
  year         = {2026},
  howpublished = {NutriDex --- The Supplement Research Compendium},
  url          = {https://nutridex.info/s/vitaminc},
  note         = {Reviewed by Dr Daryl Peh, MBBS Singapore, MMed FM. Accessed 2026-06-26}
}

For medical claims, citing the underlying primary studies linked above is preferred. NutriDex is an educational reference, not medical advice.

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