The recent review of research that pooh-poohed vitamin C for colds isnt the last word. Some researchers argue that the dosage used in many of the studies reviewed by the Cochrane Collaboration was too low (500 milligrams), and that because you lose a lot of C when you urinate, you need repeated dosing to knock out a cold.
“Taking C early on can help reduce the severity of colds, but its not clear why,” says Mary L. Hardy, MD, medical director of the Simms/Mann–University of California, Los Angeles, Center for Integrative Oncology. She recommends taking 500 mg twice a day at the first sign of a cold and continuing for five to seven days. In fact, studies show that among people who are under physical stress (marathon runners, skiers, and those living in very cold climates) 1,000 mg of C daily cuts the incidence of colds in half.