In 1970, scientist Linus Pauling believed the common cold could be controlled with vitamin C, aka L-ascorbic acid. He recommended doses of 3,000 milligrams of the vitamin supplement as the way to prevent catching a cold, and also, over time, to help completely wipe out the offending viruses.
But he was incorrect. Vitamin C is no more effective against an upper respiratory infection than a placebo is, concludes study after study. While one 2007 study found that yes, perhaps high doses of vitamin C -- we're talking vitamin C megadoses of at least 200 mg per day (more than double the average recommended daily allowance for adults) -- may ever-so-slightly reduce the length of your cold, it's only effective for a small number of people (about 8 percent of adults). If you must treat your cold with vitamin C, don't go overboard; more than 2,000 milligrams of vitamin C in a day can irritate your stomach and cause other problems, such as kidney stones. The exception: Marathon runners and other endurance athletes may find vitamin C supplements between 250 milligrams and 1 gram per day may reduce their risk of catching a cold by as much as 50 percent [source: NIH - ODS].