- Respiratory infection casesNo clear difference6% lower risk1 review of trials2022 study ↗
- Respiratory infection durationSlight effectslightly better than the comparison group1 review of trials2022 study ↗
- Respiratory infection severityNo clear differencenegligible standardised effect1 review of trials2022 study ↗
supplement
Vitamin C
Also known as Ascorbic Acid
Water-soluble antioxidant vitamin and essential cofactor for collagen synthesis and non-haem iron absorption; supports immune cell function and skin integrity, and cannot be made by the human body.
Reported side effects:Gastrointestinal disturbance - diarrhoea, nausea, abdominal cramps, stomach pain and flatulence with high-dose supplements (NHS: doses above 1,000 mg/day; NIH ODS: high intakes)
Be careful if: some medical situations, pregnant, breastfeeding
Ask first if you take: chemotherapy, statins
Upper limit: 2000 mg — per day for adults (IOM Tolerable Upper Intake Level as reported by NIH ODS; NHS separately advises 1,000 mg/day or less is unlikely to cause harm)
Suspect a side effect? Report it to the MHRA Yellow Card scheme ↗
2linked conditions
18linked studies
Evidence
What the research actually shows
We link to relevant studies and leave interpretation to you and your healthcare professional.
What studies found
The headline results, condition by condition
The large figures are what the studies reported for each thing they measured. Open a card for every result.
Colour keyReported a benefitNo clear benefitUnclearPossible harmAmber is for highlights, such as where the most research is. It never means a result was good.
- Sun damage improvementReported a benefitmeasurement not reportedTopical ascorbic acid improved photodamage compared to placebo in a small study.1 review of trials2005 study ↗
Safety
Safety and considerations
What sources report about side effects, who should be careful, and medicines to ask about first.
Reported side effects
| Reported effect | How often | Basis | Sources |
|---|---|---|---|
| Gastrointestinal disturbance - diarrhoea, nausea, abdominal cramps, stomach pain and flatulence with high-dose supplements (NHS: doses above 1,000 mg/day; NIH ODS: high intakes) | unknown | Listed by NHS (conditions/medicines pages) | Vitamin C - NHS vitamins and minerals (page last reviewed 2020-08-03)Vitamin C (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12) |
Be careful if you…
| Applies to | What sources say | Sources |
|---|---|---|
| some medical situations | NIH ODS notes concerns around high vitamin C intakes, such as kidney-stone formation and excess iron absorption, but states these are not generally considered a risk in healthy individuals. | Vitamin C (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12) |
| pregnant | No pregnancy-specific caution found in the sources checked (NHS vitamin C page lists none; NHS advises 1,000 mg/day or less from supplements is unlikely to cause harm). | Vitamin C - NHS vitamins and minerals (page last reviewed 2020-08-03) |
| breastfeeding | No breastfeeding-specific caution found in the sources checked (NHS vitamin C page lists none). | Vitamin C - NHS vitamins and minerals (page last reviewed 2020-08-03) |
Medicines to ask about
| Severity | Medicine | Sources |
|---|---|---|
| ask first | chemotherapy — Vitamin C supplements may interact with radiation therapy and some medicines, including chemotherapeutic agents (NIH ODS); people on cancer treatment should ask their oncology team before supplementing. | Vitamin C (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12) |
| listed | statins — Vitamin C supplements may interact with some medicines, including statins (NIH ODS). | Vitamin C (NIH ODS fact sheet, Health Professional edition; retrieved via Internet Archive snapshot 2026-10-12) |
How much is too much
2000 mg — per day for adults (IOM Tolerable Upper Intake Level as reported by NIH ODS; NHS separately advises 1,000 mg/day or less is unlikely to cause harm)
Evidence analytics
Where the evidence is concentrated
Audited published research for Vitamin C by condition. Drill into a pair page for the full breakdown.
Vitamin C: findings by condition
What this means: Bars count reported findings, not treatment effectiveness or certainty.
One study may report several findings or appear in more than one pairing. Counts do not show the size of an effect. Comparable numerical estimates are not established for this summary.
View the full data table
| Compound and condition | Findings (count) | Studies (count) | Reported results (finding counts) | How sure? |
|---|---|---|---|---|
| Immune Support | 3 | 1 | Benefit: 1; no benefit: 2; harm: 0; unclear: 0; not recorded / unrecognised: 0 | Not formally graded |
| Skin Health | 1 | 1 | Benefit: 1; no benefit: 0; harm: 0; unclear: 0; not recorded / unrecognised: 0 | Not formally graded |
Suggested uses (unproven)
Where people report using it and what trials suggest
- Strong evidenceScurf prevention / deficiency
- Moderate evidenceCold duration modestly shortened (~8%) in adults
- Moderate evidenceCold incidence reduced only in extreme physical stress (marathon/skiing)
Where the evidence is negative
Common claims the trials do not support
- Does not prevent colds in the general population.
- No mortality/CVD benefit in large supplement trials.
- Sepsis, cancer, and COVID outpatient evidence inconclusive.
Essential vitamin; deficiency correction is definitive, but popular cold and disease-prevention uses are weak.
Evidence grades reflect study quality and consistency, not marketing claims.
18studies referenced
12journals
2005–2026publication range
Ordered by strength of evidence, then result direction.
01Surviving Sepsis Campaign International Guidelines for the Management of Sepsis and Septic Shock in Children 2026Weiss SL, et al. · Pediatric critical care medicine : a journal of the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies · 2026PMID 41869844 ↗02Surviving Sepsis Campaign International Guidelines for the Management of Sepsis and Septic Shock in Children 2026Weiss SL, et al. · Intensive care medicine · 2026PMID 41870559 ↗03Granulomatous Reactions From Microneedling: A Systematic Review of the LiteratureFriedmann DP, et al. · Dermatologic surgery : official publication for American Society for Dermatologic Surgery [et al.] · 2025PMID 39584690 ↗04Therapeutic effects of high-dose vitamin C supplementation in patients with COVID-19: a meta-analysisSun L, et al. · Nutrition reviews · 2024PMID 37682265 ↗05Nutritional interventions for preventing and treating pressure ulcersLanger G, et al. · The Cochrane database of systematic reviews · 2024PMID 38345088 ↗06Cosmeceuticals for antiaging: a systematic review of safety and efficacyLau M, et al. · Archives of dermatological research · 2024PMID 38758222 ↗Show all 18 studies
07Microneedling as an adjuvant to topical therapies for melasma: A systematic review and meta-analysisBailey AJM, et al. · Journal of the American Academy of Dermatology · 2022PMID 33857549 ↗08Effect of Vitamin C Supplements on Respiratory Tract Infections: A Systematic Review and Meta-AnalysisKeya TA, et al. · Current reviews in clinical and experimental pharmacology · 2022PMID 34967304 ↗09ESPEN micronutrient guidelineBerger MM, et al. · Clinical nutrition (Edinburgh, Scotland) · 2022PMID 35365361 ↗10Select Dietary Supplement Ingredients for Preserving and Protecting the Immune System in Healthy Individuals: A Systematic ReviewCrawford C, et al. · Nutrients · 2022PMID 36364865 ↗11Drugs for preventing lung cancer in healthy peopleCortés-Jofré M, et al. · The Cochrane database of systematic reviews · 2020PMID 32130738 ↗12Could Vitamins Help in the Fight Against COVID-19?Jovic TH, et al. · Nutrients · 2020PMID 32842513 ↗13Vitamin C for preventing and treating tetanusHemilä H, et al. · The Cochrane database of systematic reviews · 2013PMID 24226506 ↗14Interventions for photodamaged skinSamuel M, et al. · The Cochrane database of systematic reviews · 2005PMID 15674885 ↗15Application of Antioxidant Mixture Prevents Cutaneous "Oxinflammaging" in Subjects Exposed to Particulate MatterIvarsson J, et al. · Journal of cosmetic dermatology · 2025PMID 40642809 ↗16The Effects of Dietary Supplementation with Collagen and Vitamin C and Their Combination with Hyaluronic Acid on Skin Density, Texture and Other Parameters: A Randomised, Double-Blind, Placebo-Controlled TrialŽmitek K, et al. · Nutrients · 2024PMID 38931263 ↗17A Collagen Supplement Improves Skin Hydration, Elasticity, Roughness, and Density: Results of a Randomized, Placebo-Controlled, Blind StudyBolke L, et al. · Nutrients · 2019PMID 31627309 ↗18Protective Effects of Topical Vitamin C Compound Mixtures against Ozone-Induced Damage in Human SkinValacchi G, et al. · The Journal of investigative dermatology · 2017PMID 28219661 ↗
Published research results
Reported results grouped by condition. Values and units are preserved as recorded; missing information is not estimated.
How to read this table
- Benefit
- What the trial saw happen: whether the measured outcome moved in the helpful direction in that one study (the tag on each row: “Benefit seen”, “No clear benefit”, “Possible harm” or “Unclear”). It is a study result — not advice, and not a promise it will work for you.
- Evidence
- Study design describes how research was conducted. It is not a formal certainty assessment.
- Effect size
- The result recorded in this summary. “Not reported” means the value is missing from the summary; it does not establish that the publication omitted it.
Reported direction, numerical results and study design are separate. Certainty has not been formally graded in this table.
Immune Support (3 outcomes)
| What was measured | Reported findings | Result and units | Reported sample and duration | Study / source |
|---|---|---|---|---|
| duration of respiratory infection | Vitamin C significantly shortened infection duration compared to placebo (SMD -0.36, P=0.01). (95% CI -0.62 to -0.09, P = 0.01)Benefit seen | -0.36 Recorded unit: Not reported | Sample (as reported): Not reported Follow-up: Not reported | Meta-analysis · 2022 Study detailsEffect of Vitamin C Supplements on Respiratory Tract Infections: A Systematic Review and Meta-Analysis. Meta-Analysis |
| incidence of respiratory illness | Vitamin C showed no significant benefit in preventing respiratory illness compared to placebo (RR 0.94, P=0.09). (95% confidence interval of 0.87 to 1.01, P= 0.09)No clear benefit | 0.94 Recorded unit: Not reported | Sample (as reported): Not reported Follow-up: Not reported | Meta-analysis · 2022 Study detailsEffect of Vitamin C Supplements on Respiratory Tract Infections: A Systematic Review and Meta-Analysis. Meta-Analysis |
| severity of respiratory illness | Vitamin C had no consistent effect on severity compared to placebo (SMD 0.14, P=0.09). (95% CI -0.02 to 0.30: I2 = 22%, P=0.09)No clear benefit | 0.14 Recorded unit: Not reported | Sample (as reported): Not reported Follow-up: Not reported | Meta-analysis · 2022 Study detailsEffect of Vitamin C Supplements on Respiratory Tract Infections: A Systematic Review and Meta-Analysis. Meta-Analysis |
Skin Health (1 outcome)
| What was measured | Reported findings | Result and units | Reported sample and duration | Study / source |
|---|---|---|---|---|
| Photodamage improvement with topical ascorbic acid | Topical ascorbic acid improved photodamage compared to placebo in a small study; the abstract gives no effect size.Benefit seen | Not reported Recorded unit: Not reported | Sample (as reported): Not reported Follow-up: Not reported | Meta-analysis · 2005 Study detailsInterventions for photodamaged skin. Meta-Analysis |
Important: This page reflects observational, aggregate-only community reporting and independent published research. It is not medical advice, and it does not claim any compound treats, cures, or is effective for any condition. Always consult a qualified healthcare professional before starting, stopping, or changing any supplement or treatment.
Outcomes & conditions
Outcomes & conditions →Why people look into Vitamin C — and what they report
Associated conditions and community-reported outcomes, wrapped together on one page. Aggregates appear only once enough reports arrive.
Important: This page reflects observational, aggregate-only community reporting and independent published research. It is not medical advice, and it does not claim any compound treats, cures, or is effective for any condition. Always consult a qualified healthcare professional before starting, stopping, or changing any supplement or treatment.
Availability
Where to get Vitamin C (UK)
How this compound is sold in the UK, and where our readers typically find it.
Retailer search links — we may earn commission on purchases at no cost to you. Commission never affects our evidence ratings.