Many people reach for their Vitamin C when they start feeling sick, but the science doesn't really support this idea. In a 2007 review of current literature, researchers at The Australian National University and The University of Helsinki found that Vitamin C supplementation had little to no effect on reducing the length or severity of a cold.
Vitamin C supplementation increased dramatically in the 1970's after Linus Pauling's "Vitamin C and the Common Cold" was published. In the book, he proposes that high doses of Vitamin C can prevent the common cold, yet Pauling never published any scientific research on the topic.
Bottom line? Taking Vitamin C can still benefit the system, there have been some excellent results with regard to Vitamin C and cancer prevention, but it won't help you get rid of the sniffles.
ScienceDaily (July 18, 2007) — Unless you run marathons, you probably won't get much protection from common colds by taking a daily supplemental dose of vitamin C, according to an updated review of 30 studies.