You might want to read the following:
http://www.elpasotimes.com/apps/pbcs...603200320/1004

From the article:

"The "clinically proven" statements touted in the ads allude to the results of a small double-blind trial which imply that fewer cold symptoms develop among Airborne users than the placebo group. Keep in mind that this "research" has never been peer-reviewed and details on the study have yet to be released. However, its labeling states "this product is not intended to diagnose, treat, cure or prevent any disease."

The fact is that none of its ingredients, including echinacea, and vitamins A and C, has been scientifically proved to prevent colds. Moreover, Airborne, taken as directed on the package, which is one tablet every three hours, contains large and potentially dangerous levels of vitamins A and C. For instance, a mere three tablets a day, has 15,000 units of vitamin A, enough to possibly cause liver disease and dizziness and blurred vision. High doses of A are also dangerous for pregnant women, and over the long term could increase the risk of osteoporosis. Though each tablet contains 1 g of vitamin C, doses that exceed 1 g increase oxalate and urate excretion and may cause kidney stones. Consequently, after several years of criticism, the directions now read "up to three times per day as necessary."