At the first signs of a cold, many of us pour a big glass of OJ on the assumption that 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 that increasing vitamin C intake will help cut down on sick days. Douglas RM, Hemilä H, Chalker E. The Cochrane database of systematic reviews, 2007, Jul.;(3):1469-493X. Still, the research isn't conclusive. 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. Johnston CS, Martin LJ, Cai X. Journal of the American College of Nutrition, 1992, Jun.;11(2):0731-5724. Sasazuki S, Sasaki S, Tsubono Y. European journal of clinical nutrition, 2006, Apr.;60(1):0954-3007. 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. 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. 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? 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 (), 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 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 . 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. Takkouche B, Regueira-Méndez C, García-Closas R. Epidemiology (Cambridge, Mass.), 2002, Mar.;13(1):1044-3983. Eby GA. Medical hypotheses, 2009, Nov.;74(3):1532-2777. 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 of the vitamin each day). 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 ) 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 , 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.

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