Some people swear by vitamin C or echinacea. But there is very little proof that vitamin C has any effect on the average person with a common cold. Studies have shown that very high doses of vitamin C may reduce your chance of getting a cold, but only under certain circumstances. High doses of vitamin C can hurt the kidneys and can cause nausea and diarrhea.
Echinacea is one of the best-selling herbal products in the U.S., but many researchers believe there is no proof that it has a benefit for people with colds.