Megadose vitamin C in treatment of the common cold: a randomised controlled trial

In reply: Precisely because of the temporal variation in symptom severity described by Vitetta and colleagues, we judged that medical professionals are not as well able, in a variably timed interview, to quantify patients' cold symptoms as the patients themselves can do on a continuing basis. Therefore, we consider that our study1 would have been no more valid if the detailed symptom severity cards had been supplemented by one or more physical examinations. In that respect, we are in good company with others who have studied the common cold over many years.2

We agree that double-blind randomisation does not necessarily distribute all relevant variables equally. That is why, in Box 2 of our study report, we presented four variables — age, sex, mean number of colds in the previous year, and mean number of days unwell with colds in the previous year.1 The likelihood that stress, smoking and alcohol status would have been sufficiently maldistributed in this large group to mask a significantly beneficial effect in even one of the three groups which received high-dose vitamin C seems vanishingly small. Nevertheless, we acknowledge that the study would have been stronger if we could have reported the distribution of these three potential confounders.

We contest the view of Vitetta and colleagues that the evidence from randomised controlled trials of vitamin C in treating the common cold conflicts significantly (see Box 1 of our article1). The overview finding — that mega-doses of vitamin C for prophylaxis produce a relatively trivial reduction in cold severity but no reduction in incidence3 — was the stimulus for our own study. No community studies of this issue have been flawless, but the mounting collective evidence suggests that we should look elsewhere for a cold panacea.