Common colds: Can vitamin C prevent or relieve them?

Taking vitamin C every day does not prevent colds for most people. People will not recover from a cold more quickly if they start to take vitamin C when the symptoms of a cold start. Taking vitamin C every day could shorten the amount of time you are sick with colds by a very small amount.

You need a minimum amount of vitamins to stay healthy and well, and to build up your body's defence systems. Most people get enough vitamins from their daily diets. Vitamin C (ascorbic acid) is found in fruits and vegetables, for example, especially in citrus fruits and berries.

Despite this, many people take extra vitamin C supplements every day in the hope that it will protect them against illnesses - especially the common cold. Some of these supplements have more than 1g (1000mg) of vitamin C in them - so-called mega-doses. That is more than 10 times the recommended daily dose. Because the body cannot store vitamin C, the excess vitamin usually leaves the body in urine within a few hours. In very large quantities vitamin C can also cause diarrhoea. This can cause problems, especially in small children and older people. An overdose of vitamin C can also interfere with the accurate measurement of levels of substances in the blood, such as glucose.

To find out whether taking large doses of vitamin C has more pros than cons, researchers from the Cochrane Collaboration analysed the results of trials including more than 11,000 children and adults. The doses were 2g of vitamin C daily in most of the trials.

The review showed that, even at such high doses, vitamin C could not protect most of the people from getting colds. Even when adults and children had been taking vitamin C every day before getting ill, it made hardly any difference to how long their colds lasted. The effect was very small: Overall, it reduced the length of participants' colds by less than one day per year. If adults took vitamin C once they already had a cold it did not make any difference at all.

The effect may be different in people who are exposed to short periods of really strenuous activity and/or extreme cold, like marathon runners or skiers. Research has shown that high doses of vitamin C might be able to prevent this group of people from getting colds.

There have been no reports of any noticeable adverse effects of vitamin C.


Author: German Institute for Quality and Efficiency in Health Care (IQWiG)