Vitamin C. It appears that for the most part taking vitamin C wont help the average person prevent colds. However, taking vitamin C at the onset of cold symptoms may shorten the duration of symptoms.
Echinacea. Studies on the effectiveness of echinacea at preventing or shortening colds are mixed. Some studies show no benefit. Others show a significant reduction in the severity and duration of cold symptoms when taken in the early stages of a cold. One reason study results have been inconclusive may be that the type of echinacea plant and the preparation used from one study to the next have varied considerably. Research on the role of echinacea in treating the common cold is ongoing. In the meantime, if your immune system is healthy and you are not taking prescription medications, using echinacea supplements is unlikely to cause harm.
Zinc. The cold fighting reputation of zinc has had its ups and downs. Thats because many zinc studies both those that find the mineral beneficial and those that do not are flawed. In studies with positive results, zinc seemed most effective taken within 24 hours of the onset of symptoms. Taking zinc with food may reduce side effects, including a bad taste and nausea.