Despite the popular belief that vitamin C can cure the common cold, the scientific evidence for this is conflicting.
Large doses of vitamin C may help reduce the duration of a cold, but they do not appear to protect against one in the first place, even after exposure to a cold virus.
Vitamin C may only be useful in case of a cold if you have low levels of this nutrient to begin with. For example, the vitamin may be useful for preventing a cold if you live in very low temperature environments, or you are routinely involved in vigorous exercise such as marathon running.
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 nonabsorption or urination.
Colds and vitamin C