Q. Does the dietary supplement Airborne really prevent and treat colds — even on plane flights?

A. Airborne, which appeared on Oprah Winfrey's show, is popular in Hawai'i. Each pill contains vitamin A, 1,000 milligrams of vitamin C and vitamin E, along with riboflavin, magnesium, zinc, selenium, manganese, potassium, maltodextrin, lonicera, forsythia, schizonepeta, ginger, Chinese vitex, isatis root, echinacea, glutamine and lysine.

There's no hard evidence that Airborne works — the company claims a study has been done on its product but has not released the results.

While there are studies that suggest that some echinacea preparations can help prevent and treat the common cold, most of the studies are not of high quality. The tally in favor of echinacea may be because of something known as publication bias: studies showing that echinacea works are more likely to be published than studies disproving the herbal remedy's efficacy.

In one such study, reported in the December 2002 Annals of Internal Medicine, the University of Wisconsin-Madison randomly treated 148 students with new colds with either echinacea or a placebo six times a day the first day, then three times a day for nine days. The students taking echinacea fared no better than those who did not. The highly regarded Cochrane Database of Systematic Reviews feels the evidence is far from compelling. And the amount of echinacea in Airborne is unknown.

What about vitamin C? Per the Cochrane Database, if you look at only the most accurate studies, namely, randomized studies, we find that taking it daily even at high doses does little to prevent or treat colds. When high doses of vitamin C are taken at the beginning of a cold, it does not reduce the length or severity of colds. Similarly, studies have found little evidence that zinc reduced the length of colds, but it did cause nausea and diarrhea.

None of Airborne's other ingredients have been found in reputable studies to be effective against colds, either alone or in combination. They may even prove toxic. Chinese vitex, for instance, has been linked to increases in blood pressure. High vitamin A doses are dangerous in pregnancy and can cause nausea and dizziness. And randomized studies indicate that when taken regularly in higher doses, beta-carotene (which the body converts to vitamin A) can increase incidence of heart disease and vitamin E can increase death rates. Kaiser therefore no longer sells vitamin E in doses above 100 IU.

So if you're planning a trip, forget the Airborne and rent the headsets instead.

Dr. Landis Lum is a family-practice physician for Kaiser Permanente and an associate clinical professor at the University of Hawai'i's John A. Burns School of Medicine. Send questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.