The Cochrane review was limited to placebo-controlled trials in which at least 0.2 g of vitamin C was used per day.2 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.2 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 review2, 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.
Although the regular supplementation trials demonstrate that vitamin C has physiologic effects on the common cold, taking supplements throughout the year to slightly shorten colds does not seem reasonable. Consequently, therapeutic vitamin C supplementation soon after the onset of cold symptoms seems more rational. Few trials have been conducted, and results are not consistent; methodologic variation partially explains the divergence in results.2,5,6
The only trial that compared regular and therapeutic supplementation was conducted in adults.3 The researchers administered 3 g of vitamin C per day and found no difference between regular and therapeutic supplementation.3–5 Furthermore, they found that 6 g of vitamin C per day was associated with twice as much benefit as the 3 g per day dose.3–5 Because no trials have been conducted in children, our review concluded that such trials are warranted.2
Although there is no direct evidence to show that therapeutic vitamin C would affect colds in children, and therapeutic trials with adults are only partly positive,2–6 it may still be reasonable to suggest testing vitamin C to treat colds. The results of a controlled trial are an average for a group. Vitamin C is inexpensive and safe, and its effect on an individual may be much more (or much less) than the benefit suggested by a single trial, or by the pooled results of a meta-analysis.6