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Vitamin C

This much-touted cure-all, found in citrus fruits, berries, broccoli, and green peppers, just doesn't make the grade when it comes to common-cold prevention. One study did suggest that taking vitamin C regularly might reduce the length of a cold by a day.

Bottom line: Try to get enough vitamin C through your diet. It's fine to take a supplement, especially if you're a smoker or nonsmoker who is often exposed to secondhand smoke. But there seems to be little point in upping your intake to combat sniffling and coughing.

Next: Vitamin D