Common colds: Can vitamin C prevent or relieve them?

Taking vitamin C every day to try to prevent colds does not protect most people from colds. It only slightly shortens the amount of time that they are ill. Starting to take vitamin C at the first sign of a cold does not affect the course of the cold.

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 in their daily diets. Vitamin C (ascorbic acid) is found in things like fruits and vegetables, especially in citrus fruits and berries. Medical conditions caused by a lack of vitamin C, such as scurvy in the Middle Ages, are extremely rare nowadays.

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 (1,000mg) 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.

To find out whether taking large doses of vitamin C has more pros or cons, researchers from the Cochrane Collaboration looked for randomised controlled trials on this topic. In this kind of trial, people are divided into strictly comparable groups. You can read about how randomised controlled trials are done and why they are important here. The Cochrane Collaboration is an international network of researchers who systematically review trials that test the benefits of health care interventions. The Cochrane researchers analysed the results of 29 trials involving more than 11,000 children and adults. The dose used in most of the trials was 2,000 milligrams of vitamin C per day.

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. Their colds were a bit milder too. 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. Previous research has shown that high doses of vitamin C could actually prevent this group of people from getting colds. Researchers have offered various theories to explain this. For example, vitamin C might have a protective effect against stress or stress-related infections in the lungs. But these theories have not been proven.

Adverse effects were not more common in people who took vitamin C than in people who took a placebo. The researchers consider vitamin C doses of up to several thousand milligrams per day to be safe. However, the German Institute for Risk Assessment says that getting 100 milligrams of vitamin C in your diet per day is enough.


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

© IQWiG (Institute for Quality and Efficiency in Health Care)