Dr Harri Hemila MD PhD co author of the Cochrane Review article replies :
The Cochrane review was limited to placebo-controlled trials in which at least 0.2 g of vitamin C was used per day. Most of these trials examined vitamin C administration as regular supplementation and provided strong evidence that vitamin C shortens the duration of colds and alleviates its symptoms. Children benefited more than adults. The data also suggested that high doses of vitamin C are more beneficial than low doses.[2-6]
Stratification of the regular supplementation trials in children by vitamin C dosage shows a tendency for dose dependency. Four trials, using 0.20 to 0.75 g of vitamin C per day, found an average reduction of 7 percent in common cold duration (95% confidence interval [CI], -19 to 5). Six trials with 1 g of vitamin C per day found an average reduction of 18 percent (95% CI, -32 to -3), and two trials using 2 g of vitamin C per day found an average reduction of 25 percent (95% CI, -50 to 0.1). Therefore, the 13.6 percent estimate for common cold reduction we calculated in the Cochrane review , based on all 12 trials with children who received at least 0.2 g of vitamin C per day, may underestimate the effect of high doses.
Extracted from a letter published in The American Academy of Family Physicians Oct 15th 2007 View letter
1. Simasek M, Blandino DA. Treatment of the common cold. Am Fam Physician 2007;75:515-20.
2. Douglas RM, Hemilä H, D’Souza R, Chalker EB, Treacy B. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev 2004;(4):CD000980.
3. Karlowski TR, Chalmers TC, Frenkel LD, Kapikian AZ, Lewis TL, Lynch JM. Ascorbic acid for the common cold. A prophylactic and therapeutic trial. JAMA 1975;231:1038-42.
4. Hemilä H. Vitamin C, the placebo effect, and the common cold: a case study of how preconceptions influence the analysis of results. J Clin Epidemiol 1996;49:1079-84.
5. Hemilä H. Vitamin C supplementation and common cold symptoms: factors affecting the magnitude of the benefit. Med Hypotheses 1999;52:171-8.
6. Hemilä H. Do vitamins C and E affect respiratory infections? [Academic Dissertation] University of Helsinki, Helsinki, Finland, January 2006:21-7, 36-45, 48-9, 62-3. Accessed August 6, 2007: view paper