Vitamin C may reduce the length and severity of a cold, but it doesn’t appear to prevent colds in most people, according to the National Center of Complementary and Alternative Medicine, part of the National Institutes of Health. The exception is marathon runners and other extreme exercisers, as well as people exposed to very cold weather, such as skiers. For these groups, taking 250 milligrams of vitamin C daily may reduce the chance of catching a cold by 50 percent, reports the NIH’s Office of Dietary Supplements. Studies show that using zinc lozenges within 24 hours of catching a cold can reduce the length and severity of cold symptoms.

Zinc and vitamin C may prevent vision loss in older people by delaying the progression of age-related macular degeneration, or AMD. In one study reported by the Office of Dietary Supplements, high intakes of zinc and antioxidants, including vitamin C, reduced the risk of AMD in elderly people. In another study, taking zinc alone significantly reduced the risk of developing AMD in high-risk subjects, but not in other people. Additionally, studies show high intakes of vitamin C may protect against cataracts, although the ODS notes this association needs further study.