Ascorbic acid (water-soluble vitamin)
Vitamin C
Vitamin C cures and prevents scurvy, and that is the one use nobody disputes. For colds, the Cochrane review found that taking it daily does not stop most people catching one, but shortens colds slightly (8% in adults, 14% in children); it did halve colds in people under short bursts of extreme physical stress. Applied to skin as an L-ascorbic acid serum or cream it improved photoageing and pigmentation in small trials, and high-dose supplements above about 1 g a day cause diarrhoea and were linked to more kidney stones in men.
At a glance
Last reviewed 30 September 2026Evidence
StrongBest for
Correcting deficiency (scurvy)
Typical form & use
Oral 200 mg+/day; serum 5–20% topical
Onset
4+ weeks
Safety
Over 1 g/day: diarrhoea, and more stones in men. 1 known interaction, most serious with blood thinners.
Check with your medications →What it's used for
Evidence is rated per use. The same ingredient can be strong for one thing and traditional for another.
| Use | Evidence | What the research shows |
|---|---|---|
| Correcting deficiency (scurvy), oral | Strong | Severe deficiency causes scurvy, which is potentially fatal: easy bruising and bleeding, poor wound healing, joint pain, and hair and tooth loss. It is cured by vitamin C, and as little as 10 mg a day prevents it. It is rare in wealthy countries but occurs in children, critically ill people and older adults on very restricted diets. We rate this 4 on the strength of long-established clinical knowledge; we did not cite a modern trial, because none is needed to know that it works.9,10 |
| Shortening colds, oral daily use | Moderate | The Cochrane review (31 comparisons, 9,745 cold episodes) found that regular daily vitamin C, at 0.2 g a day or more, cut cold duration by 8% in adults (95% CI 3% to 12%) and 14% in children (7% to 21%); in children, 1 to 2 g a day shortened colds by 18%. Severity was also lower. The gain is small. Taking vitamin C only after symptoms start showed no consistent effect on duration or severity in 7 comparisons.1 |
| Photoageing and hyperpigmentation, topical | Moderate | A systematic review of 7 randomised trials (139 volunteers) found treated skin looked smoother and less wrinkled, with biopsy support, and was measurably lighter; hydration improved as much on placebo. A 6-month double-blind trial of a 5% cream on photoaged skin found significant improvement in clinical score and skin relief against the base cream. In melasma, 5% ascorbic acid was rated less effective than 4% hydroquinone by patients (62.5% versus 93% good or excellent) but colorimetry showed no difference, with far fewer side effects (1 of 16 versus 11 of 16). These are small, short trials of different formulations, and the review calls for more work on the best concentration.5,6,7,8 |
| Preventing colds, oral | Limited | In 10,708 people in general-community trials, regular vitamin C gave a pooled risk ratio for catching a cold of 0.97 (95% CI 0.94 to 1.00), which is no meaningful prevention. In five trials of 598 marathon runners, skiers and soldiers on subarctic exercises the risk ratio was 0.48 (0.35 to 0.64). The Cochrane authors conclude routine supplementation is not justified but it may help people exposed to brief periods of severe exercise. The rating of 2 reflects that mixed picture.1 |
| Iron absorption from food, oral | Limited | Vitamin C makes the iron in plant foods (non-haem iron) easier to absorb, and this is strong in single test meals. In a study of 12 people eating a complete diet for 5 days, absorption did not differ significantly across daily dietary vitamin C intakes of 51 to 247 mg, though a regression still linked more vitamin C with more absorption. The authors say the effect from a whole diet is far less pronounced than from single meals, which may explain why long-term supplementation has had negligible effect on iron status.2 |
| Kidney stones (harm signal), high-dose oral | Limited | Not a use, a warning. In cohorts of 40,536 men and 156,735 women followed for about 11.5 years, total vitamin C intake of 1,000 mg or more a day carried a hazard ratio of 1.43 (95% CI 1.15 to 1.79) for kidney stones in men, and supplements of 1,000 mg or more a day 1.19 (1.01 to 1.40). No significant link was seen in women, or with vitamin C from food. This is observational, from food-frequency questionnaires; the European Food Safety Authority's panel found no increased stone risk at habitual intakes of 1.5 g a day, while noting uncertainty.4,3 |
How much
Ranges below are what studies used and what supplements commonly contain. They are not a recommendation for you. Start at the low end.
Studied for colds
0.2 g/day or more; 1–2 g/day in children1
The Cochrane review only included trials using at least 0.2 g a day. The 18% shorter colds in children were seen at 1 to 2 g a day.
Recommended intake (US)
The RDA for adults, with 35 mg a day more for smokers. The NHS says UK adults aged 19 to 64 need 40 mg a day.
Upper limit (US)
2,000 mg/day for adults9
Set to avoid diarrhoea and gastrointestinal upset. The Linus Pauling Institute says those predisposed to kidney stones may want to avoid 1 g a day or more.
EFSA's view
No upper limit set; about 1 g/day supplemental3
The EU panel judged data insufficient to set a tolerable upper level. It found supplements up to about 1 g a day on top of diet were not linked to gastrointestinal effects, and that effects may occur at 3 to 4 g a day.
Timing · No source we read gave timing advice for tablets. In the cold trials vitamin C was taken every day; starting only when symptoms began showed no consistent benefit.With food? · For iron, vitamin C helps most when taken in the same meal as plant-based iron, though the effect is smaller across a whole diet. Absorption of vitamin C itself saturates at high doses, so intakes above 1 g a day add little uptake but more gastrointestinal risk (EFSA).How long to trial · Skin trials ran 12 weeks to 6 months, so give a serum at least 3 months before judging it. Serum was applied at night in the melasma trial, alongside daily sunscreen. We found no trial comparing morning and evening use.
How people take it
Tablet or capsule (ascorbic acid)200 mg–1 g a dayThe oral form in the cold trials. Doses above 1 g a day raise the chance of diarrhoea and stomach pain (NHS, EFSA).
Food (citrus, peppers, kiwi and so on)40–90 mg a day meets needsEnough to prevent scurvy and meet the recommended intake. The kidney-stone cohorts found no link with vitamin C from food.
L-ascorbic acid serum5–20%Skin levels rise with concentration up to about 20%, and absorption needs an acidic formula (pH below about 3). It is unstable: it oxidises with light and air and turns yellow,. The pH figure comes from a review, not a trial.
Cream (5% ascorbic acid)Applied daily for 3–6 monthsThe cream used in the photoageing and melasma trials. Well tolerated in the 6-month trial.
Safety
Who should avoid
- Men with past kidney stones — supplements of 1 g a day or more were linked to more stones
- Warfarin — large doses may weaken it; ask your clinician before taking over 1 g a day
- Kidney impairment — vitamin C can increase aluminium absorption from aluminium antacids
- Regular intake above 1 g a day — diarrhoea and stomach pain; long-term safety not assessed
- Broken or very sensitive skin — stinging, redness and dryness reported with serums
- Children — upper limits are lower (for example 1,200 mg a day aged 9 to 13)
Known interactions
MonitorBlood thinners — Large doses may block the action of warfarin and lower its effectiveness, though the evidence is controversial; the Linus Pauling Institute advises limiting vitamin C to under 1 g a day and monitoring prothrombin time.
Questions people ask
What are the benefits of vitamin C?
The undisputed one is preventing and curing scurvy. Beyond that, daily supplements shorten colds by about 8% in adults and 14% in children, and topical vitamin C improved photoageing and pigmentation in small trials. It does not prevent colds in the general population, so routine supplementation is not justified by the Cochrane review.
Does vitamin C help with colds?
Not to prevent them for most people: 10,708 people in community trials had a risk ratio of 0.97. It halved colds in 598 marathon runners, skiers and soldiers under heavy physical stress. Taking it daily shortens colds slightly; starting it after symptoms begin showed no consistent effect.
Does a vitamin C serum work for skin, and how do I use it?
A systematic review of 7 trials found smoother, less wrinkled and lighter skin, but the studies were small and used different products, and long-term use may be needed. Trials used 5% creams for 3 to 6 months and melasma patients wore sunscreen daily. A stable L-ascorbic acid serum is acidic and yellows as it oxidises; stinging and redness are rare but reported, so patch test first.
What are the side effects of vitamin C, and how much is too much?
The main effect is gastrointestinal: diarrhoea, stomach pain and wind above about 1 g a day (NHS, EFSA). The US upper limit is 2 g a day; EFSA did not set one. High supplemental doses were linked to more kidney stones in men in a large cohort.
Sources
- Systematic review Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013.
- Clinical study Cook JD, Reddy MB. Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet. Am J Clin Nutr. 2001.
- Regulator opinion Scientific Committee on Food / EFSA NDA Panel. Tolerable upper intake levels for vitamins and minerals (vitamin C opinion, adopted 2004). EFSA. 2006.
- Cohort study Ferraro PM, Curhan GC, Gambaro G, Taylor EN. Total, dietary, and supplemental vitamin C intake and risk of incident kidney stones. Am J Kidney Dis. 2016.
- Review Telang PS. Vitamin C in dermatology. Indian Dermatol Online J. 2013.
- Systematic review Correia G, Magina S. Efficacy of topical vitamin C in melasma and photoaging: a systematic review. J Cosmet Dermatol. 2023.
- Randomised controlled trial Humbert PG, Haftek M, Creidi P, et al. Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo. Exp Dermatol. 2003.
- Randomised controlled trial Espinal-Perez LE, Moncada B, Castanedo-Cazares JP. A double-blind randomized trial of 5% ascorbic acid vs. 4% hydroquinone in melasma. Int J Dermatol. 2004.
- Review Linus Pauling Institute, Oregon State University. Vitamin C. Micronutrient Information Center (reviewed June 2025).
- Health service guidance NHS. Vitamin C. Vitamins and minerals.
Last reviewed 30 September 2026 · Next review March 2027 · How we rate evidence
Reader notes
Experiences and questions from readers, read by an editor before they appear. They are personal accounts, not evidence, and are not medical advice.