A large review of placebo-controlled trials of vitamin C for cold prevention and treatment has concluded that it is largely ineffective. In 30 trials involving 11,350 participants who took at least 200 milligrams of vitamin C a day, researchers found no reduction in the incidence of common colds. Vitamin C did reduce the duration and severity of cold symptoms slightly, but the effect was so small as to be clinically insignificant. The review was published last week in The Cochrane Database of Systematic Reviews. But vitamin C is apparently effective under certain circumstances. In six trials in people exposed to extreme physical stress — including ultramarathon runners, skiers and Canadian soldiers during sub-Arctic winter exercises — vitamin C reduced cold incidence by about half. "We did find an 8 percent reduction in length of colds in adults and a 13 percent reduction in children," said Dr Harri Hemila, an associate professor in the department of public health at the University of Helsinki and a co-author of the review, "so there may be greater effect with higher doses." Even in very large doses, vitamin C appears to do little harm. In 2,490 people who took more than 1,000 milligrams a day, only 5.8 percent reported any adverse symptoms, compared with 6 percent of those taking a placebo. None of the symptoms was serious. The current recommended dietary allowance for vitamin C is 90 milligrams a day for men and 75 milligrams for women. "Vitamin C is very safe," Hemila said. "The main question is whether it's useful at all and in what conditions."

New York Times