The Common Cold and Complementary Health Approaches: What the Science Says

Several randomized controlled trials and systematic reviews and meta-analyses have been conducted to examine the effects of zinc on the common cold.

• A 2015 meta-analysis of three randomized trials on zinc acetate lozenges for the common cold (dosages of 80–92 mg/day) found that zinc acetate lozenges shortened the duration of many common cold symptoms such as nasal discharge, nasal congestion, sneezing, sore throat, cough, and muscle ache. However, no differences in duration of headaches and fever were seen. The authors concluded that zinc acetate lozenges (at doses of about 80 mg/day) may be a useful treatment for the common cold, started within 24 hours of symptom onset, for a time period of less than 2 weeks.
• A 2013 review found that use of oral zinc reduces the duration of colds when taken within 24 hours of symptom onset and that prophylactic use of oral zinc reduces the incidence of colds, school and work absences, and antibiotic prescriptions. A 2012 systematic review and meta-analysis of 17 randomized controlled trials involving a total of 2,121 participants found similar results.
• A 2012 review of the treatment of the common cold in pediatric patients concluded that evidence supports the use of oral zinc as a possible treatment and prevention of the common cold in children, although its use may be limited by the need for frequent administration and potential side effects, including bad taste, throat irritation, nausea, and diarrhea.

• Oral zinc can cause nausea and other gastrointestinal symptoms.
• Long-term use of zinc, especially in high doses, can cause copper deficiency and may increase the risk of urinary tract problems, reduce immune function, and cause other side effects.
• Zinc may interact with drugs, including antibiotics and penicillamine.
• The intranasal use of zinc can cause anosmia (i.e., loss of the sense of smell), which may be long-lasting or permanent. In 2009, the U.S. Food and Drug Administration warned consumers to stop using several intranasal zinc products marketed as cold remedies because they had been linked to cases of anosmia.

Several randomized controlled trials and a few reviews, including a Cochrane review, have examined the effects of vitamin C for treating or preventing the common cold.

• A 2013 Cochrane review of results from 29 clinical trials involving more than 11,000 people found that taking vitamin C regularly (at least 0.2 grams per day) did not reduce the incidence of colds in the general population but may be useful for people exposed to brief periods of severe physical exercise (e.g., marathon runners, skiers, and soldiers training in subarctic conditions). Some studies have shown that vitamin C is associated with modest reductions in the length and severity of cold symptoms; however, these results were not replicated in the few therapeutic trials that have been conducted.
• A 2012 review of treatment of the common cold in children and adults found that prophylactic vitamin C modestly reduces the duration of cold symptoms, but not the incidence of colds, in both children and adults.

• Vitamin C is generally considered safe; however, high doses can cause digestive disturbances such as diarrhea, nausea, and abdominal cramps.

Several reviews, including a Cochrane review, of many randomized controlled trials mostly in adults have examined the effects of echinacea for preventing or treating colds.

• A 2016 systematic review and meta-analysis found that echinacea might have a preventative effect on the incidence of upper respiratory tract infections, but it is unclear whether the effect is clinically meaningful. In addition, there was no evidence that echinacea changed the duration of upper respiratory tract infections.
• A 2014 Cochrane review of 24 double-blind randomized controlled trials involving 4,631 participants concluded that echinacea products have not been shown to provide benefits for treating colds. Although there is the potential that some preparations are more effective than placebo for treating colds, the overall evidence for clinically relevant treatment effects is weak. This same review also concluded that the results of individual prophylaxis trials consistently show positive (though not significant) trends, although potential effects are of questionable clinical relevance.
• A 2013 review concluded that therapeutic use of Echinacea purpurea may improve cold symptoms in adults, but the evidence is inconsistent. The review also concluded that prophylactic use of echinacea preparations is ineffective for preventing the common cold. The effects of echinacea in children are uncertain; only a small amount of research has been done in children, and the results of that research are inconsistent.

• Few side effects have been reported in clinical trials of echinacea; however, some people may have allergic reactions.
• In a 2003 randomized controlled trial conducted in children, use of Echinacea purpurea was associated with an increased risk of rash.

Several randomized controlled trials on a variety of strains and doses, as well as reviews and meta-analyses, have been conducted to examine the effects of probiotics on the common cold.

• A 2015 Cochrane review of 13 randomized controlled trials involving 3,720 participants including children, adults, and older adults found that probiotics (any specified strain or dose) were better than placebo in reducing the number of participants experiencing episodes of acute upper respiratory tract infections (URTI), the mean duration of an episode of acute URTI, antibiotic use, and cold-related school absences. These findings suggest that probiotics may be more beneficial than placebo for preventing URTIs; however, the quality of the evidence was low or very low.
• A 2014 systematic review and meta-analysis examined the effectiveness of probiotics on the duration of illness in healthy children and adults who develop common acute respiratory infections and found evidence from “good-quality” randomized controlled trials that probiotics reduce the duration of illness in otherwise healthy children and adults.

• As pointed out in a 2011 Agency for Healthcare Research and Quality assessment of the safety of probiotics, little is known about the long-term effects of probiotics.
• It appears that most people can use probiotics without experiencing any side effects—or with only mild gastrointestinal side effects such as gas—but there have been some case reports of serious adverse effects, and the risk of serious side effects may be greater in people with underlying health conditions. Probiotics should not be used by people with serious underlying health problems except with close monitoring by a health care provider.

Several randomized controlled trials in both children and adults, as well as a recent Cochrane review, have been conducted to examine the effects of saline nasal irrigation on the common cold.

• A 2015 Cochrane review of five randomized controlled trials involving 544 children and 205 adults found that saline nasal irrigation possibly has benefits for relieving the symptoms of acute upper respiratory tract infections; however, the trials included in the review were generally too small and had a high risk of bias.
• A 2012 review in children concluded that during acute illness, saline nasal irrigation can help alleviate sore throat, thin nasal secretions, and improve nasal breathing, and can reduce the need for nasal decongestants and mucolytics.

• Nasal irrigation is generally safe; however, neti pots and other rinsing devices must be used and cleaned properly.
• Most important is the source of water that is used with nasal rinsing devices. According to the U.S. Food and Drug Administration, tap water that is not filtered, treated, or processed in specific ways is not safe for use as a nasal rinse. Some tap water contains low levels of bacteria and protozoa, including amoebas, which can stay alive in nasal passages and cause potentially serious infections.
• Improper use of neti pots may have caused two deaths in 2011 in Louisiana from a rare brain infection that the state health department linked to tap water contaminated with an amoeba called Naegleria fowleri.

There have only been a couple of small studies in children that have examined the effects of honey for symptoms of the common cold.

• A 2010 study, involving 139 children aged 2 to 5 years, compared the effect of honey, dextromethorphan, and diphenhydramine on nighttime cough and sleep quality and found that receiving a 2.5-mL dose of honey before sleep had a more alleviating effect on upper respiratory infection-induced cough compared with dextromethorphan and diphenhydramine doses.
• A 2007 study of 105 children aged 2 to 18 years with upper respiratory infections compared honey to similar pharmacologic agents and found that parents rated honey most favorably for symptomatic relief of their child’s nighttime cough and sleep difficulty due to upper respiratory tract infection.

• Honey should not be used in children younger than 1 year of age because of the risk of botulism.

Several randomized controlled trials in adults and in children, and a recent Cochrane review, have been conducted to examine the effects of geranium extract on the common cold.

• A 2013 Cochrane review of eight randomized controlled trials involving adults and children concluded that Pelargonium sidoides may be effective in relieving symptoms in acute bronchitis in adults and children, and sinusitis in adults. Also, Pelargonium sidoides may be effective in alleviating symptoms of acute rhinosinusitis and the common cold in adults, but there is some doubt. The overall quality of the evidence was considered low for main outcomes in acute bronchitis in children and adults, and very low for acute sinusitis and the common cold.
• A 2007 double-blind, randomized controlled trial in 103 adults with common colds found that Pelargonium sidoides (administered in liquid drops 3 times a day) significantly reduced the severity of symptoms and shortened the duration of the cold compared with placebo.

• Research suggests that Pelargonium sidoides is generally well tolerated in most people.

Only a few clinical trials have been conducted to examine the effects of garlic supplements on the common cold.

• A 2014 Cochrane review of a single randomized controlled trial, involving 146 participants who received either a garlic supplement or a placebo for 12 weeks, concluded that there is insufficient clinical trial evidence regarding the effects of garlic in preventing or treating the common cold. The single trial suggested that garlic may prevent occurrences of the common cold but more studies are needed to validate this finding.

• Garlic is probably safe for most people in the amounts usually eaten in foods.
• Taking garlic supplements may increase the risk of bleeding. Patients on anticoagulants should use caution when considering garlic supplements.
• Garlic has been found to interfere with the effectiveness of some drugs, including saquinavir, a drug used to treat HIV infection.

A small number of studies in people have evaluated elderberry for flu and other upper respiratory infections; however, conclusive evidence from high-quality clinical trials is lacking.

• A 2020 review of studies concluded that although there is a hypothetical benefit of elderberry for the treatment of viral infections, there isn’t enough conclusive evidence from high-quality clinical trials.
• A 2019 meta-analysis of four studies found that supplementation with black elderberry (Sambucus nigra) was significantly effective at reducing the total duration and severity of upper respiratory symptoms, compared with placebo.
• A 2016 double-blind, randomized controlled trial evaluating elderberry in air travelers suggested that elderberry may reduce cold symptom duration and severity.

• Raw unripe elderberries and other parts of the elder tree, such as the leaves and stem, contain toxic substances (e.g., sambunigrin) that can cause nausea, vomiting, and diarrhea; cooking eliminates this toxin. Large quantities of the toxin may cause serious illness.
• Little is known about whether it’s safe to use elderberry during pregnancy or while breastfeeding.