A large number of placebo-controlled studies have shown that vitamin C supplementation alleviates the symptoms of the common cold, but widespread skepticism that vitamin C could have any significant effect remains. One of the most influential common cold studies, published in 1975, was carried out by Thomas Karlowski et al, at the National Institutes of Health. Their placebo consisted of lactose, which can easily be distinguished from ascorbic acid by taste. Karlowski et al, found a 17% decrease in the duration of cold episodes in the group administered vitamin C (6 g/day); however, they suggested that the decrease was entirely due to the placebo effect. In this article it will be shown that the placebo effect is not a valid explanation for the results of the Karlowski study, as it is inconsistent with their results. This is an important conclusion for two reasons. First, the placebo explanation becomes even more unreasonable as regards the reported benefits found in several other studies with valid placebo tablets. Second, as the results from the Karlowski study are not due to the placebo effect, their results can be used to assess the quantitative effects of vitamin C supplementation. The most important conclusions from Karlowski's study are that therapeutic vitamin C supplementation during a common cold episode appears to be as effective as regular supplementation, and that there appears to be linear dose dependency at least up to 6 g/day. These findings suggest that large therapeutic vitamin C doses might alleviate the symptoms of the common cold substantially.