But for just as long, scientists have been going back and forth on whether the vitamin is effective at preventing or treating the common cold.

Vitamin C was first isolated in the 1930s, and studies investigating its potential to prevent colds got underway in the 1940s. In 1970, two-time Nobel Prize-winning chemist Linus Pauling touted the powers of C in his best-selling book, "Vitamin C and the Common Cold." He was particularly inspired by a 1961 study at a ski school in the Alps. Kids in the study who took one gram of vitamin C per day (the U.S. Food and Drug Administration's daily reference value is 60 milligrams a day) had 45% fewer colds than their untreated classmates. They also recovered from colds in two-thirds the time it took their peers to get better.

Pauling's book and subsequent papers on the topic encouraged numerous researchers to investigate the alleged wonder vitamin in large-scale clinical studies of their own. Soon, conflicting evidence began to emerge. Some studies found the vitamin reduced the frequency of colds, some found it reduced the duration of colds, but still others found that it had no effect at all.

Last summer, the Cochrane Collaboration, a nonprofit organization that reviews the science on health topics, reviewed more than 50 well-designed, published studies on vitamin C and the common cold. The researchers found that taking at least 200 milligrams of C on a daily basis doesn't reduce the odds of getting a cold -- but it does speed recovery time by about 8% in adults. In children, it hastens recovery by 13.6%.

The reviewers found another interesting effect. While vitamin C doesn't keep colds at bay in most cases, it cuts the risk of colds in half in people under great physical stress: marathoners, skiers and soldiers training under arctic conditions.

Some evidence also suggests that taking a very large dose of C -- up to 8 grams -- at the onset of a cold could shorten the duration of infection or lessen symptoms, but the Cochrane reviewers concluded that more research was needed to confirm this effect.

For those who aren't winter marathoners or Olympic athletes, vitamin C probably offers little in the way of cold prevention, said Penn State's Dr. Ian Paul. "But is it harmful? Probably not," he added.

High-dose vitamin C does have a slight downside: Three to 5 grams at a time can cause gastrointestinal effects, including diarrhea.