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An analysis of 29 studies including 11,306 participants concluded that supplementing with 200 mg or more of vitamin C did not reduce the risk of catching a cold. However, regular vitamin C supplements had several benefits, including:

Reduced cold severity: They reduced the symptoms of a cold, making it less severe.

Reduced cold duration: Supplements decreased recovery time by 8% in adults and 14% in children, on average. A supplemental dose of 1–2 grams was enough to shorten the duration of a cold by 18% in children, on average.

Other studies in adults have found 6–8 grams per day to be effective.

Vitamin C appears to have even stronger effects in people who are under intense physical stress. In marathon runners and skiers, vitamin C alantost halved the duration of the common cold.

SOURCE:

https://www.healthline.com/nutrition/does-vitamin-c-help-with-colds#TOC_TITLE_HDR_2