At the first signs of a cold, many of us pour a big glass of OJ on the assumption that loading up on vitamin C is a surefire way to kick just about any bug. Modern-age nose-blowers may also reach for “immunity boosters” like Airborne and Emergen-C to cure the sniffles. But does vitamin C—and the supplements that tout its benefits—really work to prevent (or cure) the common cold?

Out in the Cold—Why It Matters

Researchers have studied the role vitamin C plays in preventing and treating the common cold for more than 60 years. Most experts say there is still little proof that increasing vitamin C intake will help cut down on sick days. Vitamin C for preventing and treating the common cold. Douglas RM, Hemilä H, Chalker E. The Cochrane database of systematic reviews, 2007, Jul.;(3):1469-493X. Still, the research isn't conclusive. One study found that taking a daily vitamin C supplement reduced the frequency of catching a cold, while another discovered that it has an antihistamine effect that could reduce cold symptoms. Antihistamine effect of supplemental ascorbic acid and neutrophil chemotaxis. Johnston CS, Martin LJ, Cai X. Journal of the American College of Nutrition, 1992, Jun.;11(2):0731-5724. Effect of vitamin C on common cold: randomized controlled trial. Sasazuki S, Sasaki S, Tsubono Y. European journal of clinical nutrition, 2006, Apr.;60(1):0954-3007. Another study found that vitamin C made a big difference in preventing colds in those exposed to brief periods of intense cold or extreme physical exercise (like skiers, military personnel, and marathon runners) but not the general population. Vitamin C for preventing and treating the common cold. Douglas RM, Hemilä H, Chalker E. The Cochrane database of systematic reviews, 2007, Jul.;(3):1469-493X. And a different study suggests that upping vitamin C intake could reduce the severity and duration of a cold—and hopefully erase the need for that economy-size tissue box. Vitamin C for preventing and treating the common cold. Hemilä H, Chalker E. The Cochrane database of systematic reviews, 2013, Jan.;1():1469-493X.

So it looks like some vitamin C, which is found naturally in superfoods like oranges, bell peppers, and strawberries, certainly won't do us any harm. But what about the massive doses found in products like Airborne and Emergen-C? Created by a schoolteacher in 1997, each tablet of Airborne contains 1,000 mg of vitamin C (equivalent to 11 glasses of OJ) along with zinc, vitamins A and E, selenium, and a blend of herbs including ginger and echinacea. Emergen-C also contains 1,000 mg of vitamin C (1,667 percent of the daily recommended value) and recommends users take it up to two times daily. Each serving also includes B vitamins, zinc, and electrolytes, which is why it claims to enhance energy (without the caffeine crash). While neither of them outright say they can prevent or cure colds (anymore!), the mega doses of vitamin C are generally the reason many cold-sufferers sniffle their way to the supplement aisle.

Too Much of a Good Thing?—The Answer/Debate

While there are no product-specific studies testing Airborne and Emergen-C’s effectiveness in preventing and treating the common cold, research that looks at ingredients like vitamin C and zinc can give us some insight into how well the products work. As we've shown, the research on vitamin C is mixed, though many professionals maintain that it's not an effective treatment. Vitamin C for preventing and treating the common cold. Hemilä H, Chalker E. The Cochrane database of systematic reviews, 2013, Jan.;1():1469-493X. Research on zinc also remains pretty inconclusive: Multiple studies suggest that it is not effective at treating colds, though one study did conclude that it may be at high doses. Intake of vitamin C and zinc and risk of common cold: a cohort study. Takkouche B, Regueira-Méndez C, García-Closas R. Epidemiology (Cambridge, Mass.), 2002, Mar.;13(1):1044-3983. Zinc lozenges as cure for the common cold--a review and hypothesis. Eby GA. Medical hypotheses, 2009, Nov.;74(3):1532-2777. Zinc lozenges may shorten the duration of colds: a systematic review. Hemilä H. The open respiratory medicine journal, 2011, Jun.;5():1874-3064. Obviously more research is needed before anyone goes around touting zinc as the latest miracle cure.

So it looks like taking these immunity boosters is likely neither seriously beneficial nor harmful. But there are still a few things to consider before overdoing it on the fizzy drinks. Too much vitamin C, for example, can cause diarrhea, nausea, stomach cramps, and kidney stones (the National Institutes of Health suggest that adults consume no more than 2,000 mg of the vitamin each day). Ascorbic acid supplements and kidney stone incidence among men: a prospective study. Thomas LD, Elinder CG, Tiselius HG. JAMA internal medicine, 2013, Apr.;173(5):2168-6114.

Similarly, too much vitamin A (which is often included in these immune boosters) might do more harm than good. In excess doses (defined by the NIH as more than 10,000 IU per day) can cause dizziness, nausea, headaches, coma, and (in rare cases) death. In other words, it is possible to have too much of a good thing. So when it comes to catching a bug, it’s probably best to save money on the hype and listen to the classic recommendations: get lots of sleep, keep your hands clean, and cook some chicken soup.

The Takeaway

While regularly consuming adequate amounts of vitamin C may help reduce the frequency of catching colds, there's little evidence that it can actually help prevent or treat sickness once it's already set in.

Originally published December 2011. Updated February 2016.

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