In 2013, Professor Harri Hemilä, together with Elizabeth Chalker, did the most comprehensive review of all vitamin C studies[i], called a Cochrane systematic review, with the aim being “To find out whether vitamin C reduces the incidence, the duration or severity of the common cold when used either as a continuous regular supplementation every day or as a therapy at the onset of cold symptoms.” They excluded any studies giving less than 200 mg a day. They found that for the most part vitamin C didn’t reduce the number of colds but did reduce the severity and duration of colds, in those experiencing enhanced physical stress.
In 2017, Harri Hemilä revisited the studies, paying more attention to the question of dose. He reported that “Two controlled trials found a statistically significant dose – response, for the duration of common cold symptoms, with up to 6-8 g per day of vitamin C. Thus, the negative findings of some therapeutic common cold studies might be explained by the low doses of 3-4 g/day of vitamin C.” What he found was that the more vitamin C that was given on the first day of a viral infection, the better were the results.
He also found that while there were not a lesser number of colds if you combined all studies “Vitamin C has prevented colds in British men”.[ii] Four trials found that vitamin C decreased the incidence of colds by 30%, and in another set of four trials, the proportion of men who had recurrent common cold infections during the study decreased by a mean of 46%. That’s almost a halving in number of colds. Vitamin C also halved the incidence of colds in five randomised controlled trials during which the participants were under heavy short-term physical activity. A more recent small study in the US reported a 45% decrease in the number of colds through supplementation in those with low vitamin C levels.[iii] Then, two other studies reported that 16% and 17% more of those children taking vitamin C versus placebo didn’t succumb to infection.[iv] If you put this the other way around, one in six children taking vitamin C didn’t get a cold.
Hemilä also shows that, if the dose is higher (4 g to 8 g per day) the duration is shorter. In two studies giving placebo, 3-4 g versus 6-8 g on the first day, the duration of colds was 10% shorter with 3-4 g and 20% shorter with 6-8 g on the first day. In those taking 8 g on the first day, 46% had symptoms that only lasted for one day.[v]
What happens if you take it for longer? In a five day study only giving 1.5 g on the first day and to 1 g of vitamin C thereafter there was a 25% reduction in duration.
What happens if you give more, and take it for longer? In 1999 a study involving 463 students age 18 to 32, were given either 1 g of vitamin C every hour for the first six hours of a cold or flu, then 3 g a day thereafter, or pain relievers and decongestants.[vi] “Flu and cold symptoms in the test group decreased 85% compared with the control group.” Those taking vitamin C halved the duration of the cold, from 7 days to 3.5 days, compared to those taking cold medication. (This study tested the groups over different winters so they weren’t necessarily exposed to the same cold viruses.)
[i] H Hemilä and E Chalker, ‘Vitamin C for preventing and treating the common cold.’ Cochrane Database of Systematic Reviews, (2013). [https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD000980.pub4/abstract]
[ii] H Hemilä, ‘Vitamin C intake and susceptibility to the common cold.’ British Journal of Nutrition, (1997), 77:59-72. [https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0007114500002889]; see also C Bates et al, ‘Vitamin C intake and susceptibility to the common cold: Invited comments and Reply.’ British Journal of Nutrition, (1997), 78:857-66; see also H Hemilä, ‘Vitamin C and common cold incidence: A review of studies with subjects under heavy physical stress.’, International Journal for Sports Medicine, (1996), 17:379-83. [https://www.thieme-connect.com/products/ejournals/abstract/10.1055/s-2007-972864]
[iii] C Johnston, ‘Vitamin C supplementation slightly improves physical activity levels and reduces cold incidence in men with marginal vitamin C status: a randomized controlled trial.’ Nutrients, (2014), 6(7):2572-83. [https://www.mdpi.com/2072-6643/6/7/2572]
[iv] J Coulehan et al, ‘Vitamin C prophylaxis in a boarding school.’ New England Journal of Medicine, (1974), 290(1):6-10. [https://www.nejm.org/doi/full/10.1056/NEJM197401032900102]; see also M Van Straten and P Josling, ‘Preventing the common cold with a vitamin C supplement: a double-blind, placebo-controlled survey.’ Advances in Therapy, (2002), 19(3):151-9. [https://link.springer.com/article/10.1007/BF02850271]
[v] H Hemilä, ‘Vitamin C and Infections.’, Nutrients, (2017), 9(4):339. [https://www.mdpi.com/2072-6643/9/4/339]
[vi] H Gorton and K Jarvis, ‘The effectiveness of vitamin C in preventing and relieving the symptoms of virus-induced respiratory infections.’ Journal of Manipulative and Physiological Therapeutics, (1999), 22(8):530-3.[https://www.ncbi.nlm.nih.gov/pubmed/10543583]