Do popular supplements work for the common cold?

The common cold is a viral infection of the upper respiratory system.  On average, children have six to eight colds per year, and adults have two to four. The common cold places a heavy burden on society, accounting for approximately 40% of time taken off work and millions of days of school missed by children each year.  While medical science has yet to develop a cure for the common cold, many people take a variety of supplements in the hopes that they will shorten the duration of the illness. The most popular of these supplements are zinc, vitamin C, and Echinacea.
  
Let's look at some of the scientific evidence of the effectiveness of these supplements. Much of this information comes from the Cochran Group, an international network of scientists who compile the work of several researchers into larger studies known as "meta-analyses".  By pooling data from a number of well-conducted studies, more robust and accurate conclusions can be drawn.

Zinc is commonly sold as a natural medicine for colds in tablet, lozenge, and liquid form.  It is thought to decrease the ability of cold viruses to grow on or bind to the lining of the nose. The results of studies have gone back and forth over the years regarding zinc's effectiveness in treating colds. Based on a 2011 analysis of clinical trials, The National Center for Complementary and Integrated Health reports that "oral zinc helps to reduce the length and severity of colds when taken within 24 hours after symptoms start."  After initially supporting the benefits of zinc in treating the common cold, the Cochrane Group is currently re-evaluating existing research and has not released its final opinion on its effectiveness.

Because the dosages and duration of treatment with zinc varied across the various studies evaluated, the optimal dosage for zinc supplementation has not been established.  Before taking a zinc supplement, people should be aware that it has the potential to cause nausea and other gastrointestinal side effects. In 2009, the U.S. Food and Drug Administration advised consumers to stop using intranasal zinc products) because of reports of anosmia (loss of smell).

 Echinacea is one of the most commonly used herbal supplements.  Advocates say that it is can boost the immune system and has antiviral properties, such as preventing colds.  However, two 2006 meta-analyses evaluating this natural remedy drew conflicting conclusions.  In the first analysis, extracts of Echinacea were found to be somewhat effective in preventing cold symptoms if someone had been clinically inoculated with the cold virus. The second, a Cochrane Systematic Review, found no evidence that Echinacea was any better than a placebo at treating the common cold once symptoms had begun.  Likewise, three clinical trials funded by the National Center for Complementary and Integrated Health found no benefit from Echinacea for preventing or treating colds. Echinacea products vary widely, with different preparations using different species of the plant as well a different plant parts.

 Vitamin C is an important micronutrient and antioxidant. The role of vitamin C in the prevention and treatment of the common cold has been a subject of controversy for decades. Nevertheless, it is widely used as both a preventive as well as therapeutic supplement.  A 2013 Cochrane analysis of results from 29 clinical trials involving 11,306 participants found the following related to vitamin C and prevention or treatment of colds:

1. "The failure of vitamin C supplementation to reduce the incidence of colds in the general population indicates that routine vitamin C supplementation is not justified."
    
2. "Trials of high doses of vitamin C administered after the onset of symptoms, showed no consistent effect on the duration or severity of common cold symptoms."
    
3. "Given the consistent effect of vitamin C on the duration and severity of colds in the regular supplementation studies, and the low cost and safety, it may be worthwhile for common cold patients to test on an individual basis whether therapeutic vitamin C is beneficial for them."
    
4. "Vitamin C supplementation may be useful for people exposed to brief periods of severe physical exercise (e.g. running a marathon) or to cold environments. In those people, taking vitamin C appeared to cut their risk of catching a cold in half."

The final word regarding these and other supplements in the prevention and treatment of the common cold awaits further research. In regard to preventing colds, the best methods currently available appear to be "behavioral" in nature, such as washing your hands regularly, getting plenty of rest, limiting exposure to people with colds, and avoiding touching your nose or eyes with potentially contaminated hands.