When you feel a sore throat, a runny nose, or a cough coming on, immediately take zinc. This mineral is one of the most intensively studied and proven cold fighters. There's just one catch: You have to take it within the first 24 hours after symptoms start. Aim for 75 milligrams daily, or about six lozenges' worth over the course of a day, says Dr. David Agus, a professor at the University of Southern California and author of 'A Short Guide to a Long Life.' Echinacea has shown promise in shortening the length of a cold, but because it is an herbal supplement, it's not strictly regulated and quality varies widely. Vitamin C, on the other hand, just doesn't work – or at least the evidence that it can prevent or slow a virus is very weak. Less effective are those fizzing citrus supplements: "Vitamin C degrades when it starts to evanesce," says Agus, so you won't get any benefits at all.