While you can get all the vitamin C you need from foods to help you prevent the common cold, you might need to supplement with this vitamin once you feel a cold coming on. A few studies have verified that while vitamin C might not prevent the cold from happening, it will help curb its severity and duration. Begin supplementing with 250 milligrams or more at the first signs of a cold. That’s only for adults; young children are much more susceptible to toxicity effects from vitamins and minerals, so keep their intake to within recommended levels or discuss higher doses with your physician.