If the onset of a cold has you reaching for the vitamin C bottle, you may want to think twice. Vitamin C, an essential water-soluble vitamin not stored in the body, has a reputation as an immunity booster that can help fight off viruses. This may be true, but possibly only under certain conditions and at certain doses. Generally, taking vitamin C once you are sick does not help reduce the duration or symptoms of illness.
At what point you take vitamin C may have some impact on whether it helps with the symptoms or duration of a cold or other virus. A July 2007 "Cochrane Database of Systematic Reviews" review of 30 studies examined the duration and severity of viruses in participants who took at least 200 mg of vitamin C a day prophylactically. A reduction of cold duration equal to 8 percent in adults and 13 percent in children occurred when subjects took vitamin C before they became ill. A Japanese study published in the January 2006 "European Journal of Clinical Nutrition" found similar results, with subjects taking 500 mg per day experiencing a 66-percent reduced risk of developing three or more colds than subjects taking 50 mg.
Once You are Sick
In subjects who started vitamin C after symptoms started and who did not take daily vitamin C supplements, no benefit in either the duration or the severity of symptoms was noted in the "Cochrane" review. This review looked at seven different clinical studies that assessed the duration of symptoms and four studies that looked at the severity of symptoms.
Linus Pauling, the biochemist whose work on vitamin C first sparked interest in its use in treating viruses in 1970, advocated large doses, sometimes called megadoses, of vitamin C of up to 1 g, or 1,000 mg per day. Critics of available studies state that doses used in available studies may not have been high enough to show benefit, according to the Linus Pauling Institute.
Current studies suggest that taking vitamin C once you develop a cold or virus has no benefit. MayoClinic.com does state that a small reduction in cold duration has been reported, but the percentages are too insignificant to warrant a recommendation. The Mayo Clinic rates evidence of a positive effect as a "D," meaning that scientific evidence does not support the use of taking vitamin C after a cold or virus begins.