Despite the popular belief that vitamin C can cure the common cold, the scientific evidence for this is conflicting. A few studies have suggested that taking vitamin C supplements when you are exposed to a cold virus or when you first have symptoms can shorten a cold or even prevent one. Other research has shown that vitamin C supplementation has no impact on a cold's severity or length, but it can significantly reduce how often a person catches a cold.

Vitamin C may only be useful in case of a cold if you have low levels of this nutrient to begin with. The likelihood of success may be very individual -- some people improve, while others do not.

People with kidney disease should avoid vitamin C supplements. Most experts advise that you meet your daily vitamin and mineral requirements by eating a balanced diet. Taking more than 500 mg of vitamin C at any one time provides no advantage. More than that amount is simply lost through non-absorption or urination.
    - "Colds and vitamin C"

... In a subset of studies in people living in extreme climates or under extraordinary conditions, including soldiers in sub-arctic exercises, skiers, and marathon runners, vitamin C significantly reduced the risk of developing colds by approximately 50%.

This area merits more study and may be of particular interest to elite athletes or military personnel. - "Common cold prevention (extreme environments)"

Just what constitutes an optimum dose isn't clear, but amounts in excess of 2,000 mg a day may cause nausea and diarrhea.
    - "Cold remedies: What probably doesn't hurt"