Vitamin C

What it is

Vitamin C (ascorbic acid) is a water-soluble vitamin that the human body cannot make, so it has to come from food or supplements [1]. Supplements usually contain plain ascorbic acid, whose bioavailability is similar to the form naturally present in foods [1]. Vitamin C is an antioxidant and is needed for normal immune function [1].

How it might work

Proposed mechanisms: vitamin C concentrates in immune cells such as leukocytes, and low leukocyte vitamin C is linked with reduced immune cell function [1]. Adequate cell and tissue stores of vitamin C are thought to help maintain resistance to infections, and its antioxidant activity is the other suggested route [1]. Which of these explains any effect on colds is not established [2].

What the research says

Shorter, milder colds with daily use. In the 2013 Cochrane review, regularly taking vitamin C shortened colds by about 8% in adults (95% CI 3-12%) and 14% in children (7-21%); children given 1-2 g/day had colds 18% shorter, and severity was also reduced [2]. A 2023 meta-analysis of 10 double-blind trials (doses at least 1 g/day) found vitamin C cut cold severity by 15% (9-21%), with a larger effect on severe symptoms than mild ones [3]. In 28 men with low vitamin C status, 1 g/day for 8 weeks was linked to fewer colds and a 59% shorter duration that was not statistically significant [4].

Preventing colds. In general community trials, daily vitamin C did not reduce cold incidence (risk ratio 0.97, 95% CI 0.94-1.00; 10,708 participants) [2], and a 2018 overview of systematic reviews agreed that it does not prevent colds [5]. In a cohort of 21,796 male smokers, dietary vitamin C was not linked to cold frequency [6]. Under extreme physical stress the picture changes: five trials of 598 marathon runners, skiers and soldiers on subarctic exercises found about half as many colds (risk ratio 0.48, 95% CI 0.35-0.64) [2], and a 2022 trial of 1,444 Korean army recruits found a 20% lower risk of a diagnosed cold with 6,000 mg/day over 30 days [7].

Starting at symptom onset. Seven therapeutic comparisons (3,249 cold episodes) found no consistent effect on duration or severity [2], and a 2018 rapid review reported minimal or no impact on duration or days off [8]. In 39 adolescent swimmers, 1 g/day did not change infection rates but reduced cold duration by 47% in boys and not in girls [9].

Doses used in studies

The cold trials included doses of at least 0.2 g/day (lower doses were excluded from the review), with many studies using 1-2 g/day; children's colds were 18% shorter at 1-2 g/day [2]. The 2023 severity analysis included only trials giving at least 1 g/day [3]. The army recruit trial used 6,000 mg/day for 30 days [7]; the swimmer trial and the low-status men's trial used 1 g/day for 3 months and 8 weeks respectively [9][4]. Tested supplement forms include plain ascorbic acid and calcium ascorbate [1].

Safety and interactions

Common side effects. Unabsorbed ascorbic acid commonly causes gastrointestinal effects - nausea, heartburn, epigastric pain, abdominal cramps and diarrhea - because of its acidity and osmotic effect in the intestine [1]. In one 10-day study, people sensitive to acidic foods who took plain ascorbic acid at 1,000 mg/day rising to 2,000 mg/day reported more abdominal pain (p = 0.02) and diarrhea (p = 0.047) than those taking calcium ascorbate [1].

Upper limits and toxicity. The common cold trials did not establish a toxic upper dose [2]. Vitamin C supplements increase urinary oxalate by 6.45 mg/24 h versus placebo across nine studies, which may contribute to kidney stone formation [10]. In observational studies, men taking supplemental vitamin C at 1,000 mg/day or more had a higher risk of kidney stones (hazard ratio 1.19, 95% CI 1.01-1.40; pooled odds ratio in men 1.62, 95% CI 1.09-2.42), while women did not [11][12].

Interactions. Vitamin C increases iron absorption, and people with iron overload (hemochromatosis) should avoid vitamin C supplements [13].

Pregnancy and breastfeeding. In 29 randomised trials with 24,300 pregnant women, vitamin C supplementation showed no clear benefits or harms for pregnancy outcomes, and routine use was not supported [14]. No data on cold prevention in breastfeeding women were found in the studies reviewed.

Conditions. People with a history of kidney stones or iron overload should be cautious with supplements (see above) [11][13].

Caveats and open questions

The main Cochrane review dates from 2013 and no newer Cochrane update was found in our searches [2]. Treatment trials are few and inconsistent: seven comparisons found no clear benefit [2] and a 2018 rapid review found minimal or no effect [8]. Individual trials are small - the swimmer trial had 39 people and found a benefit in boys only [9], and the low-status men's trial had 28 people with a non-significant duration result [4]. Compared with zinc, vitamin C has been studied less recently: a 2024 Cochrane review of zinc included 34 trials with 8,526 participants [15].