Yes, we are still investigating this question. Cold and flu season is coming and vitamin C is often touted as a natural remedy for the common cold. Patients ask me about it every week. A 2013 look at several studies, a meta-analysis, of over 11,000 people did conclude that regular vitamin C supplementation (at least 200 mg per day) slightly reduced the duration of cold symptoms. The problem was that this was only an 8 percent reduction in the duration of cold symptoms and most people didn’t notice that difference.
What about taking high doses once I start getting sick? This question has been asked and answered in several studies and you may not like the answer. High doses of vitamin C started after symptom onset have not been shown to reduce symptom duration or severity. In addition, there is no consistent benefit of vitamin C on prevention of the common cold in the general population
But wait, some folks did benefit. An interesting finding what that there was a 50 percent decrease in the incidence of colds in a subset of patients exposed to vigorous activity, especially in extreme conditions (marathon runners and skiers). The reasons for the benefit in this group of patients are uncertain.