The bulk of scientific research on the effectiveness of vitamin C in fighting infection has centered on reducing the symptoms and duration of the common cold. Much of the studies have used doses of approximately 1 gram per day. Findings from a 2017 study suggest that larger doses might reduce a cold’s duration.

Although most adults typically have only one or two colds per year, cold symptoms are the reason for many lost days of work or school. Some evidence suggests that the financial costs associated with having a cold are similar to those associated with having high blood pressure or a stroke.

The author of the study reviewed the findings of two randomized trials focused on the effectiveness of vitamin C in reducing cold symptom duration. One of the trials had four treatment groups: one group that took a placebo, two groups that took 3 grams per day, and one group that took 6 grams per day. The 6-gram dose reduced cold symptom duration by about 17 percent – roughly twice as much as that observed with only 3 grams. The placebo had no effect on symptom duration. The other trial had three treatment groups: one that took 4 grams per day, one that took 8 grams per day, and one that took a placebo. Taking 8 grams per day reduced symptom duration by 21 percent, compared to the placebo group.

These findings suggest that large doses of oral vitamin C might reduce the duration of symptoms associated with the common cold, but self-dosing should commence as soon as cold symptoms appear for the greatest benefit.

• Link to full study.