What are natural treatments for colds and flus?
A cold is a respiratory infection caused by one of hundreds of possible viruses. However, because these viruses are so widespread, it is perhaps more accurate to say that colds are caused by a decrease in immunity that allows one of these viruses to take hold.
Colds occur more frequently in winter, but no one knows exactly why. Nearly everyone catches colds occasionally, but some people catch colds quite frequently; others tend to stay sick an unusually long time.
Influenza B, commonly called the flu, occurs in the form of a worldwide epidemic every winter. The predominant symptoms of flu are fever, malaise, and muscle aches. Coldlike respiratory symptoms are usually fairly minor with the flu, but a dangerous type of pneumonia can develop as a complication of influenza, especially in the elderly.
Conventional medicine can neither cure nor prevent the common cold. Furthermore, no over-the-counter treatments have been found to shorten the duration of a cold or even provide significant temporary relief. Cough syrup, in studies, seems to be no better than placebo. Some of the natural treatments described in this section may be able to do better.
People often want to take antibiotics for colds, and many physicians will prescribe them (even though antibiotics have no effect on viruses). Many believe that when associated mucus turns yellow, it means that a bacterial infection has occurred for which antibiotic treatment is indicated. However, viruses can also produce yellow mucus. Even if bacteria have made a home in the excess mucus, they may be only “innocent bystanders” and produce no symptoms. Colds, however, can be complicated by bacterial infections. In such cases, antibiotic treatment may be indicated.
The situation is somewhat better for influenza. The flu shot provides protection against several strains of influenza. There are also prescription antiviral medications that can help prevent flu and also reduce its length and severity.
Various natural treatments have shown promise for treating or preventing colds and flu.
Zinc. One well-known alternative treatment for colds and flu is the use of zinc in nasal gel or lozenges. When zinc is taken this way, it is not being used as a nutrient. Rather, certain forms of zinc release ions that are thought to directly inhibit viruses in the nose and throat.
Taking zinc orally as a nutrient might also be useful in some cases. The immune system does not function properly if one does not have enough zinc in his or her body. Because zinc is commonly deficient in the diet, especially among children and the elderly, nutritional zinc supplementation may certainly be useful for those who get sick easily.
A one-year double-blind study of fifty nursing home residents found that zinc supplements compared with placebo reduced rates of infection. In addition, more than ten other studies performed in developing countries have found that zinc supplements at nutritional doses can increase resistance to respiratory and other infection in children, and that they might reduce symptom severity. With zinc, more is not better; once one has enough zinc, getting extra will not help and might even be harmful.
Scientific evidence for zinc nasal gel and lozenges. The use of lozenges containing zinc gluconate or zinc acetate has shown somewhat inconsistent but generally positive results for reducing the severity and duration of the common cold. For example, in a double-blind trial, one hundred people who were experiencing the early symptoms of a cold were given a lozenge that contained either 13.3 milligrams (mg) of zinc from zinc gluconate or a placebo. Participants took the lozenges several times daily until their cold symptoms subsided. The results were impressive. Coughing disappeared within two days in the treated group versus four days in the placebo group. Sore throat disappeared after one day versus three days in the placebo group, nasal drainage in four days (versus seven days), and headache in two days (versus three days). Positive results have also been seen in double-blind studies of zinc acetate. Not all studies have shown such positive results. However, the overall results appear to be favorable.
It has been suggested that the exact formulation of the zinc lozenge plays a significant role in its effectiveness. According to this view, certain flavoring agents, such as citric acid and tartaric acid, might prevent zinc from inhibiting viruses. In addition, chemical forms of zinc other than zinc gluconate or zinc acetate might be ineffective. Zinc sulfate in particular might not work. Along the same lines, sweeteners such as sorbitol, sucrose, dextrose, and mannitol are said to be fine, while glycine has been discussed in an equivocal manner.
The use of zinc in the nose is somewhat more controversial. In addition to showing inconsistent results in studies, the use of zinc nasal gel can cause pain and possibly loss of sense of smell. In one double-blind, placebo-controlled trial, 213 people with a newly starting cold used one squirt of zinc gluconate gel or placebo gel in each nostril every four hours while awake. The results were significant: Treated participants stayed sick an average of 2.3 days, while those receiving placebo were sick for an average of 9 days, a 75 percent reduction in the duration of symptoms. Somewhat more modest but still significant relative benefits were seen with zinc nasal gel in a double-blind, placebo-controlled study of eighty people with colds. However, a slightly larger study of a similar zinc gluconate nasal gel found no benefit. Another study, this one involving seventy-seven people, failed to find benefit, even with nearly constant saturation of the nasal passages with zinc gluconate nasal spray.
Echinacea. Until the 1930s, echinacea was the number-one cold and flu remedy in the United States. It lost its popularity with the arrival of sulfa antibiotics. Ironically, sulfa antibiotics are as ineffective against colds as any other antibiotic, while echinacea does seem to be at least somewhat helpful. In Germany, echinacea remains the main remedy for minor respiratory infections.
Echinacea is generally thought to work by temporarily stimulating the immune system, although most later evidence has tended to cast doubt on this belief. Contrary to popular belief, however, there is little reason to believe that echinacea strengthens or “nourishes” the immune system when taken over the long term.
There are three main species of echinacea: E. purpurea, E. angustifolia, and E. pallida. A mixture containing all the parts of E. purpurea found above ground (flowers, leaves, stems) has the best supporting evidence for effectiveness in treating colds and flu; the root of E. purpurea is probably not effective, while the root of E. pallida may be the active part of that species.
Echinacea has shown promise for reducing the symptoms and duration of colds and aborting a cold once it has started. However, echinacea does not appear to be helpful for preventing colds. It may also not be effective in children.
Reducing the symptoms and duration of colds. Double-blind, placebo-controlled studies enrolling more than one thousand people have found that various forms and species of echinacea can reduce the symptoms and duration of a common cold. This is true for adults. In one double-blind, placebo-controlled trial, eighty persons with early cold symptoms were given either E. purpurea extract or placebo. The results showed that those who were given echinacea recovered significantly more quickly: in just six days among the echinacea group versus nine days among the placebo group.
Another double-blind, placebo-controlled trial looked at the reduction of the severity of cold symptoms. The results in 246 participants showed that treatment with E. purpurea significantly improved cold symptoms such as runny nose, sore throat, sneezing, and fatigue. Symptom reduction with E. purpurea was also seen in a double-blind, placebo-controlled study of 282 people.
In addition, three double-blind, placebo-controlled studies enrolling about six hundred persons found similar benefits with a combination product containing E. purpurea and E. pallida root, along with wild indigo and white pine.
While the evidence tends to suggest that the above-ground portion of E. purpurea is active against the common cold, two studies have failed to find benefit. One of these was a double-blind, placebo-controlled study enrolling 120 adults, the other an even larger trial (407 participants) involving children. The reason for these negative outcomes is not clear. E. angustifolia root has also failed to prove effective in a large study.
Aborting a cold. A double-blind study suggests that echinacea, in addition to making colds shorter and less severe, might also be able to stop a cold that is just starting. In this study, 120 people were given E. purpurea or a placebo as soon as they started showing signs of getting a cold.
Participants took either echinacea or placebo at a dosage of twenty drops every two hours for one day, then twenty drops three times a day for a total of up to ten days of treatment. The results were promising. Fewer people in the echinacea group felt that their initial symptoms actually developed into “real” colds (40 percent of those taking echinacea versus 60 percent taking the placebo actually became ill). Also, among those who did come down with “real” colds, improvement in the symptoms started sooner in the echinacea group (four days instead of eight days). Both of these results were statistically significant.
Preventing colds. Several studies have attempted to discover whether the daily use of echinacea can prevent colds from even starting, but the results have not been promising. In one double-blind, placebo-controlled trial, 302 healthy volunteers were given an alcohol tincture containing E. purpurea root, E. angustifolia root, or placebo for twelve weeks. The results showed that E. purpurea was associated with perhaps a 20 percent decrease in the number of people who got sick, and E. angustifolia with a 10 percent decrease. However, the difference was not statistically significant. This means that the benefit, if any, was so small that it could have come from chance alone.
Another double-blind, placebo-controlled study enrolled 109 persons with a history of four or more colds during the previous year and gave them either E. purpurea juice or placebo for a period of eight weeks. No benefits were seen in the frequency, duration, or severity of colds. Four other studies also failed to find statistically significant preventive effects.
A study often cited as evidence that echinacea can prevent colds actually found no benefit in the 609 participants taken together. Only by looking at subgroups of participants (a statistically questionable procedure) could researchers find any evidence of benefit, and it was still slight.
However, a more recent study using a combination product containing echinacea, propolis, and vitamin C did find preventive benefits. In this double-blind, placebo-controlled study, 430 children ages one to five years were given either the combination or the placebo for the months during the winter. The results showed a statistically significant reduction in frequency of respiratory infections. It is not clear which of the components of this mixture was responsible for the apparent benefits seen.
Andrographis. Andrographis is a shrub found throughout India and other Asian countries, sometimes called Indian echinacea because it is believed to provide much the same benefits. It has been used historically in epidemics, including the Indian flu epidemic in 1919, during which andrographis was credited with stopping the spread of the disease. It is now popular in Scandinavia as a treatment for colds.
Although it is not known how andrographis might work for colds, some evidence suggests that it might stimulate immunity. The ingredient of andrographis used for standardization purposes, andrographolide, does not appear to affect the immune system as much as the whole plant extract.
According to a few well-designed studies (almost all of which used the proprietary extract produced by a single company), andrographis can reduce the symptoms of colds. It may offer the additional useful benefit of helping to prevent colds.
Reducing cold symptoms. Seven double-blind, placebo-controlled studies enrolling almost one thousand people have found that andrographis (or a combination containing it as the presumed primary ingredient) significantly reduces the duration and severity of cold symptoms.
For example, a four-day, double-blind, placebo-controlled trial of 158 adults with colds found that treatment with a proprietary andrographis extract significantly reduced cold symptoms. Participants were given either placebo or 1,200 mg daily of an andrographis extract standardized to contain 5 percent andrographolide. The results showed that by day two of treatment, and even more by day four, participants given the actual treatment experienced significant improvements in symptoms compared to participants in the placebo group. The greatest response was seen in earache, sleeplessness, nasal drainage, and sore throat, but other cold symptoms improved too.
Three other double-blind, placebo-controlled studies, enrolling about four hundred people, evaluated a proprietary herbal combination treatment containing both andrographis and Eleutherococcus senticosus (Russian ginseng) and found benefit. Another study suggests that this combination may be more effective than echinacea. (Somewhat confusingly, this proprietary combination is sold under the same name, Kan Jang, as the pure andrographis product already noted; the manufacturer regards this combination as more effective than andrographis alone, and the combination version of the product has now superseded the previous single-herb version.) The same combination has also shown promise in two double-blind studies for reducing the duration, severity, and rate of complications of influenza.
Andrographis has also been compared to acetaminophen (Tylenol). In a double-blind study of 152 adults with sore throat and fever, participants received andrographis (in doses of 3 or 6 g per day for seven days) or acetaminophen. The higher dose of andrographis (6 g) decreased symptoms of fever and throat pain to about the same extent as acetaminophen, but the lower dose of andrographis (3 g) was not as effective. There were no significant side effects in either group. This study used a different form of andrographis than the proprietary product already noted.
Preventing colds. According to one double-blind, placebo-controlled study, andrographis may increase resistance to colds. A total of 107 students, all eighteen years of age, participated in this three-month trial that used the same proprietary extract of andrographis noted earlier. Fifty-four of the participants took two 100 mg tablets standardized to 5.6 percent andrographolide daily, considerably less than the 1,200 to 6,000 mg per day that has been used in studies on treatment of colds. The other fifty-three students were given placebo tablets with a coating identical to the treatment. Then, once a week throughout the study, a clinician evaluated all the participants for cold symptoms.
By the end of the trial, only sixteen people in the group using andrographis had experienced colds, compared to thirty-three of the placebo-group participants. This difference was statistically significant, indicating that andrographis reduces the risk of catching a cold by a factor of two compared with placebo.
Vitamin C. Vitamin C may mildly reduce symptoms of colds when they occur, but it probably does not help prevent colds.
Treating colds. Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg or more daily can reduce the severity of cold symptoms and shorten their duration. However, the effect is modest at best. In addition, at least one study suggests that vitamin C can enhance the effect of standard cold treatments, such as acetaminophen. According to most of these studies, using vitamin C throughout the cold season, rather than intermittently, appears to be beneficial. For example, a review of twenty-nine placebo-controlled trials involving almost one thousand episodes of illness concluded that taking a minimum of 2,000 mg per day seems to result in shorter and less severe colds when they occur. However, high doses of vitamin C do not appear to decrease the number of colds experienced during a season.
Many people use vitamin C for colds in a different way: They begin taking it only after cold symptoms first appear. In a review of seven randomized and nonrandomized trials, researchers found that this approach–taking high doses of vitamin C (for example, 2,000 to 4,000 mg) at the first sign of illness–does not seem to affect the cold’s severity or duration.
Preventing colds. Although some studies suggest that regular use of vitamin C throughout the cold season can help prevent colds, most other studies have found little to no benefit along these lines. Vitamin C has shown a bit more promise for prevention of one type of cold, the so-called postmarathon sniffle. These are colds that develop after endurance exercise; the use of vitamin C before and during competition may help keep a person cold-free afterward. In addition, vitamin C seems to help prevent respiratory infections among persons who are actually deficient in the vitamin.
Essential oils. Eucalyptus is a standard ingredient in cough drops and in oils meant to be added to humidifiers. A standardized combination of three essential oils has been tested for its usefulness in respiratory conditions. The studied combination, called essential oil monoterpenes, includes cineole from eucalyptus, d-limonene from citrus fruit, and alpha-pinene from pine. Numerous double-blind trials have found them effective when taken orally for acute bronchitis, chronic bronchitis, sinus infections, and other respiratory conditions, in both adults and children. Cineole alone at a dose of 200 mg three times daily showed benefit in a double-blind, placebo-controlled study of 152 people with cold symptoms. A second study involving 150 persons also demonstrated favorable results of cineole compared to a combination of five other herbal products.
Ginseng. Although most people in the West think of ginseng as a stimulant, in Eastern Europe ginseng is widely believed to improve overall immunity to illness. As noted, echinacea does not seem to prevent respiratory infections, but it appears that the regular use of ginseng might be able to provide this important benefit.
There are three different herbs commonly called ginseng: Asian or Korean ginseng (Panax ginseng), American ginseng (Panax quinquefolius), and Siberian ginseng (Eleutherococcus senticosus). The latter herb, which is not discussed here, is actually not ginseng.
A double-blind, placebo-controlled study of 323 people found meaningful evidence that an extract of American ginseng taken at 400 mg daily may help prevent the common cold. Participants who used the extract for four months experienced a reduced number of colds compared to those taking placebo. Comparative benefits were also seen regarding the percentage of participants who developed two or more colds and the severity and duration of cold symptoms that did develop. Similar benefits were also seen in a study of forty-three people. Two double-blind, placebo-controlled studies enrolling about one hundred people indicate that American ginseng may also help prevent flulike illness in the elderly.
Garlic. The herb garlic has a long history of use for treating or preventing colds. However, until 2001, there was no scientific evidence that it actually works for this purpose. In one twelve-week, double-blind, placebo-controlled trial, 146 persons received either placebo or a garlic extract between November and February. The results showed that participants receiving garlic were almost two-thirds less likely to catch cold than those receiving placebo. Furthermore, participants who did catch cold recovered about one day faster in the garlic group compared with the placebo group. Note that these studies do not indicate that taking garlic will help once a person already has a cold.
Probiotics. Probiotics are healthful organisms that colonize the digestive tract. Not only can they help prevent intestinal infections, they also appear to help prevent colds.
A seven-month, double-blind, placebo-controlled study of 571 children in day care centers in Finland found that the use of milk fortified with the probiotic bacterium Lactobacillus GG modestly reduced the number and severity of respiratory infections. In another controlled trial, probiotics (L. rhamnosus GG and Bifidobacterium lactis Bb-12) given daily to infants in their formula significantly reduced the risk of acute otitis media and recurrent respiratory infections during the first year of life compared with placebo. Benefits were also seen in three other large studies, in which probiotics, alone or combined with multivitamins-multiminerals, helped prevent colds or reduce their duration and severity in adults.
Another controlled trial involving twenty healthy elite distance runners found that L. fermentum given for four months during winter training was significantly more effective at reducing the number and severity of respiratory symptoms than a placebo. In addition, a small double-blind study found evidence that the use of the probiotic bacterium L. fermentum improved the effectiveness of the influenza vaccine. The probiotic supplement was taken in two doses: the first two weeks before the vaccine, and the other two weeks after.
Various other natural treatments have shown some promise for preventing or treating colds and flu.
Preventing respiratory infections. The use of multivitamin-multimineral supplements, or supplements containing zinc and selenium alone, may help prevent respiratory infections in the elderly, according to some studies. One small double-blind study suggests that the supplement arginine might be helpful for preventing colds in children.
A gargle made from green tea extract has shown promise for preventing influenza. In a double-blind, placebo-controlled study, 124 residents of a Japanese nursing home gargled with green tea catechins or placebo for three months. All participants received standard influenza vaccine. The results showed that those who gargled with the tea extract were less likely to develop influenza than those using placebo. In addition, another double-blind study found preliminary evidence that oral consumption of a green tea extract might help prevent both colds and flu.
There is some evidence that the supplement glutamine may, like vitamin C, help prevent postexercise infections. For example, a double-blind, placebo-controlled study evaluated the benefits of supplemental glutamine (5 g) taken at the end of exercise in 151 endurance athletes. The result showed a significant decrease in infections among those treated. Only 19 percent of the athletes taking glutamine got sick, compared to 51 percent of those on placebo. Echinacea has also shown some promise for this purpose.
In contrast, some evidence suggests that a combination of vitamin E and beta-carotene treatment might increase the risk of exercise-associated colds. The evidence regarding whether vitamin E taken alone can prevent respiratory infections is conflicting.
The thymus gland plays a role in immunity. A one-year, double-blind, placebo-controlled trial of sixteen children with frequent respiratory infections found that treatment with thymus extract could reduce the rate of infection. However, a double-blind, placebo-controlled trial of sixty athletes failed to find any significant evidence of benefit with thymus extract for preventing postexercise infections.
An extract of rice bran has shown some promise for preventing or treating colds in the elderly. Also, a throat spray made from sage has shown considerable promise for reducing sore throat pain. A study widely reported as showing that the supplement colostrum can help prevent colds was actually far too preliminary to prove anything. There is some evidence that elements in kelp might help to prevent infection with several kinds of viruses, including influenza. However, the evidence thus far is more theoretical than practical.
Various herbs, including ashwagandha, astragalus, garlic, maitake, reishi, and suma, are said to enhance immunity over the long term. However, there is no meaningful evidence that they really work. In addition, several herbs, including ginger, kudzu, osha, and yarrow, are said to help avert colds when taken at the first sign of infection; but again, there is no scientific evidence that they are effective.
Products containing colloidal silver are sometimes used in the belief that they will prevent colds and otherwise strengthen the immune system; however, because colloidal silver can cause permanent color changes in the skin, its use is not recommended.
Some older persons do not respond fully to the influenza vaccine. There is some evidence that vitamin E supplements may strengthen the immune response to vaccinations. Similarly, evidence from two double-blind trials, but not a third, suggests that combined multivitamin-multimineral supplements may improve their response. However, in another trial, a multivitamin tablet without minerals actually worsened participants’ response to the vaccine.
Two studies suggest that combined multivitamin-multimineral supplements can also improve immune response to the vaccine. However, two others failed to find benefit, and in one study a multivitamin tablet without minerals actually worsened participants’ responses to the vaccine. The reason for these discrepancies is unclear. In a double-blind, placebo-controlled study of 124 people, the supplement chlorella at a dose of 200 or 400 mg daily failed to enhance response to influenza vaccine. Another study failed to find benefit with a remedy from the traditional Chinese herbal medicine Hochu-ekki-to.
Treatment of respiratory infections. A standardized product containing elderberry combined with small amounts of echinacea and bee propolis has been widely marketed as a cold and flu remedy. Weak evidence suggests that this mixture may stimulate the immune system and also inhibit viral growth. In a preliminary double-blind study, the combination significantly reduced the recovery time from epidemic influenza B (a relatively mild form of influenza). Another small double-blind study found similar benefits in both influenza A and B.
One small study found that the popular Throat Coat brand of medicinal beverage teas actually does reduce sore throat discomfort, compared with placebo tea. Also, inhaled essential oils have shown some slight promise for the treatment of colds.
The herb Pelargonium sidoides is used in Europe for the treatment of colds and other respiratory infections. A double-blind study of 133 adults who had just come down with the common cold found that taking a standardized Pelargonium extract at a dose of 30 milliliters three times daily significantly reduced the severity and duration of symptoms compared with placebo.
In double-blind, placebo-controlled studies enrolling more than three hundred people, a combination of four herbs (primrose, gentian root, elderberry, common sorrel, and vervain) has shown promise for treatment of sinusitis. Another study provides weak evidence that a standardized combination of horseradish and nasturtium might be helpful for the treatment of the common cold in children. Other herbs sometimes recommended to reduce cold symptoms, but that lack meaningful supporting scientific evidence, include hyssop, ivy leaf, linden, marshmallow, mistletoe, mullein, oregano, and peppermint.
In a double-blind, placebo-controlled trial, colostrum was not helpful for people with sore throat. (The researchers were sure to exclude people with strep throat, but some participants may have had sore throat caused by bacteria rather than cold viruses.) A substantial (254-participant) double-blind, placebo-controlled study failed to find that the use of the berry of Hippophae rhamnoides (sea buckthorn) reduced the number or duration of colds. Finally, a 2009 review of seventeen trials found that there is limited evidence to support the use of traditional Chinese herbal preparations for the common cold.
Cox, A. J., et al. “Oral Administration of the Probiotic Lactobacillus fermentum VRI-003 and Mucosal Immunity in Endurance Athletes.” British Journal of Sports Medicine 44 (2010): 222-226.
Eby, G. A., and W. W. Halcomb. “Ineffectiveness of Zinc Gluconate Nasal Spray and Zinc Orotate Lozenges in Common-Cold Treatment.” Alternative Therapies in Health and Medicine 12 (2006): 34-38.
Goel, V., et al. “A Proprietary Extract from the Echinacea Plant (Echinacea purpurea) Enhances Systemic Immune Response During a Common Cold.” Phytotherapy Research 19 (2005): 689-694.
Halperin, S. A., et al. “Safety and Immunoenhancing Effect of a Chlorella-Derived Dietary Supplement in Healthy Adults Undergoing Influenza Vaccination.” CMAJ: Canadian Medical Association Journal 169 (2003): 111-117.
Hemila, H., E. Chalker, and B. Douglas. “Vitamin C for Preventing and Treating the Common Cold.” Cochrane Database of Systematic Reviews (2010): CD000980. Available through EBSCO DynaMed Systematic Literature Surveillance at http://www.ebscohost.com/dynamed.
Hubbert, M., et al. “Efficacy and Tolerability of a Spray with Salvia officinalis in the Treatment of Acute Pharyngitis.” European Journal of Medical Research 11 (2006): 20-26.
Kurugol, Z., N. Bayram, and T. Atik. “Effect of Zinc Sulfate on Common Cold in Children.” Pediatrics International 49 (2007): 842-847.
Larmo, P., et al. “Effects of Sea Buckthorn Berries on Infections and Inflammation.” European Journal of Clinical Nutrition 62 (2008): 1123-1130.
Linde, K., et al. “Echinacea for Preventing and Treating the Common Cold.” Cochrane Database of Systematic Reviews (2006): CD000530. Available through EBSCO DynaMed Systematic Literature Surveillance at http://www.ebscohost.com/dynamed.
O’Neil, J., et al. “Effects of Echinacea on the Frequency of Upper Respiratory Tract Symptoms.” Annals of Allergy, Asthma, and Immunology 100 (2008): 384-388.
Olivares, M., et al. “Oral Intake of Lactobacillus fermentum CECT5716 Enhances the Effects of Influenza Vaccination.” Nutrition 23 (2007): 254-260.
Rautava, S., S. Salminen, and E. Isolauri. “Specific Probiotics in Reducing the Risk of Acute Infections in Infancy.” British Journal of Nutrition 101 (2009): 1722-1726.
Rowe, C. A., et al. “Specific Formulation of Camellia sinensis Prevents Cold and Flu Symptoms and Enhances T Cell Function.” Journal of the American College of Nutrition 26 (2007): 445-452.
Tesche, S., et al. “The Value of Herbal Medicines in the Treatment of Acute Non-purulent Rhinosinusitis.” European Archives of Oto-Rhino-Laryngology 265 (2008): 1355-1359.
Zhang, X., et al. “Chinese Medicinal Herbs for the Common Cold.” Cochrane Database of Systematic Reviews (2007): CD004782. Available through EBSCO DynaMed Systematic Literature Surveillance at http://www.ebscohost.com/dynamed.