supplement
Evidence tier: strong
Potent antioxidant vitamin essential for immune cell function, collagen synthesis, and iron absorption, one of the most evidence backed vitamins for cold prevention
An essential cofactor. Vitamin C is required for the hydroxylation of proline and lysine that lets collagen cross link and hold its shape, which is why deficiency literally unravels connective tissue and causes scurvy. It is also a cofactor for noradrenaline and carnitine synthesis, the body's main water soluble antioxidant, and it reduces dietary iron from the ferric to the ferrous form, which is why it materially improves non haem iron absorption. Note that the cofactor mechanism explains what deficiency does, not what supplementation above sufficiency does, and the cold trials are the demonstration that the two are not the same question.
Strong, but strong for something other than what people buy it for. Vitamin C is an essential nutrient and the evidence that it prevents and cures scurvy is definitional and beyond argument. NIH ODS sets the RDA at 90 mg daily for adult men and 75 mg for adult women, plus 35 mg extra for smokers. That is the strong claim, and it is a deficiency claim, exactly like iron and vitamin B12. Now the famous claim, and the correction. The Cochrane review of vitamin C for preventing and treating the common cold (Hemila and Chalker) pooled 29 trial comparisons in 10,708 people and found that regular supplementation does NOT reduce how often you catch a cold: pooled RR 0.97, 95% CI 0.94 to 1.00. The authors' own conclusion is that the failure of vitamin C supplementation to reduce the incidence of colds in the general population indicates that routine vitamin C supplementation is not justified. Linus Pauling was wrong, and the trials that tested him say so. What survives is smaller and real: regular supplementation shortens colds by about 8% in adults and 14% in children, a consistent effect across 31 comparisons and 9,745 cold episodes, but that is roughly a day off a twelve day cold, not a cure. And starting vitamin C AFTER symptoms appear, which is what most people actually do, showed no consistent effect on duration or severity at all. There is exactly one population where the prevention claim holds: five trials in 598 marathon runners, skiers and soldiers on subarctic exercises produced a pooled RR of 0.48, roughly halving colds under extreme physical stress. Safety: NIH ODS sets a Tolerable Upper Intake Level of 2,000 mg daily. Above that, stomach upset and diarrhoea, and, less commonly, kidney stones, because excess ascorbate is excreted as oxalate. The megadose is where the risk lives, and the megadose is where the benefit does not.
RDA (NIH ODS): 90 mg daily for men aged 19 and over, 75 mg for women, plus 35 mg for smokers. Tolerable Upper Intake Level: 2,000 mg daily for adults. The Cochrane cold trials that produced the modest duration effect generally used 200 mg or more daily taken REGULARLY, not started at symptom onset. There is no sourced basis for the 1,000 to 2,000 mg immune support doses in common circulation, and doses above 2,000 mg exceed the UL.
Take 500 to 1000mg daily with food. Higher doses (2g+) do not provide additional benefit for most people and may cause digestive upset. Use liposomal form for enhanced absorption.
Onset: ~7 days
Very safe at recommended intakes. High doses (>2 g/day) may cause osmotic diarrhoea and increase kidney stone risk (oxalate) in susceptible individuals. Safe in pregnancy at RDA (85 mg/day); avoid megadosing.
Pregnancy: safe
Breastfeeding: safe
Vitamin C itself is a modern discovery, but vitamin C-rich foods have been used in traditional medicine for centuries. Rose hips, sea buckthorn, and citrus fruits were used in folk medicine to prevent scurvy and support healing. Lemon juice was prescribed by the British Navy in the 18th century for scurvy prevention, long before the vitamin was isolated.
Active compounds: ascorbic acid, dehydroascorbic acid
Remedies using Vitamin C · Browse by body system · Check it against what you take
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.