Does vitamin C prevent colds?

Researchers from the Cochrane Collaboration – an international network of researchers – looked into the question of whether taking large doses of vitamin C can protect against colds or relieve the symptoms. To find out, they analyzed studies comparing vitamin C with a product that didn't contain any active ingredients (a placebo).

29 studies, involving more than 11,000 children and adults, tested whether the regular use of supplements containing at least 200 mg of vitamin C prevented colds. Most of the studies tested a dose of 1,000 or more mg of vitamin C per day. Some of the participants took the vitamin C over a period of several years.

The studies showed that it wasn't possible to prevent colds by taking vitamin C every day over a longer period of time. But doing so did shorten the amount of time that people were ill by about 10 percent. In other words, a cold that would have lasted ten days was over in nine days. The cold symptoms were also a bit milder in people who always took vitamin C.

Other studies show that taking vitamin C only after becoming ill didn’t shorten the length of colds in men and women. It didn’t relieve their symptoms, either.

Some of the studies looked at whether vitamin C can prevent colds in people exposed to short periods of very strenuous physical activity, often in extremely cold temperatures. Examples include marathon runners or soldiers doing winter exercises in a mountainous region. The study participants started taking vitamin C two to three weeks before the activities, with the aim of preventing colds. It was found that doing so reduced their risk of developing a cold by about half.

Taking very high doses of vitamin C regularly may cause diarrhea, but the study participants who took vitamin C didn’t report having more side effects than those who used a placebo.