Relevant findings for each search term are briefly summarized in 8–37
and are detailed as follows:
Complementary and alternative medicine therapies for the common cold
Echinacea roots and flowers were used medicinally by First Nations peoples and became popular in Europe for infectious problems. In laboratory studies, they activate macrophages, increase phagocytosis, enhance cytokine production38
and natural killer cell activity, and improve lymphocyte and monocyte cell counts10
and antibody response.39
Many active ingredients have been identified, but recent attention has focused on alkylamides, which bind to cannabinoid receptors and upregulate the transcription of tumour necrosis factor.40
Clinical trial results have been difficult to interpret because they have used different species and plant parts. A recent Cochrane systematic review of echinacea for treatment and prevention of the common cold41
found more evidence for Echinacea purpurea
than Echinacea angustifolia
or Echinacea pallida
, and more for aerial parts than roots. Given this, this review is limited to studies of E purpurea
Of the 11 trials identified that studied echinacea,8–15,42–44
6 trials evaluated E purpurea
in 764 healthy adults with cold symptoms.10–15
Symptom severity was reduced in 4 of 6 trials,10–12,14
measured using total daily symptom scores and Jackson scores. Of the 4 trials that measured duration of symptoms,12–15
3 found significant reductions of 1.5,12
and 4 days13
<.05 for all). Only 1 study found no reduction in symptom severity or duration.15
The 6 trials used 5 distinct preparations: 3 tinctures11,13–15
and 2 tablets derived from various extracts.10,12
Three trials studied healthy adults,11,14,15
while the other 3 studied adults with 2 or more colds in the previous year.10,12,13
Different clinical scoring systems were also used. There is moderate evidence that E purpurea
might be effective for treatment of the common cold, but issues surrounding dose and formulation require clarification before it can be recommended for routine use.
Two trials evaluating E purpurea
in cold prevention found no reduction in symptom duration or severity.8,9
Safety and use:
A typical dose of echinacea is 2000 to 3000 mg of crude extract, 6 to 9 mL of pressed juice, or 0.75 to 1.5 mL of tincture per day. The most important concern is allergy; echinacea is a member of the Asteraceae family and can cause serious reactions in susceptible patients.45
Reported side effects include gastrointestinal upset, headache, and rash.39,46,47
No herb-drug interactions have been identified. Its safety in pregnancy or for long-term use is unknown. The theoretical risk of worsening autoimmune disease has not been reported but should be considered.39
means “heal-all” in Greek. Panax ginseng
(Asian ginseng) has been used in Chinese medicine for millennia, and is believed to be an “adaptogen,” which enhances an individual’s ability to resist mental and physical stress.48
In animal and human studies, it has activated macrophages, natural killer cells, and lymphocytes, and increased cytokine and antibody production.49,50 Panax quinquefolius
(North American ginseng) contains a similar chemical profile and is used to make COLD-fX, an extract standardized to polysaccharides and oligosaccharides.
Treatment: There are no trials evaluating ginseng for treatment of the common cold.
Our search identified 3 trials comprising 564 patients that evaluated P quinquefolius
for prevention of the common cold26–28
; all were funded by the manufacturer. These trials were also identified in a recent systematic review.51
Unfortunately, none involved healthy adults. One summarized 2 separate studies of 89 and 109 nursing home patients26
; another involved 44 elderly patients (older than 65 years of age)27
; and a third examined 323 unvaccinated adults with more than 2 colds in the previous year.31
All patients received 200 mg per day of COLD-fX or placebo for 2 to 4 months.
Poor and misleading reporting of data makes it difficult to draw conclusions from these studies. The nursing home studies reported fewer laboratory-confirmed infections, but found no reduction in the number of colds or in the duration or severity of symptoms.26
The study of 44 elderly patients reported fewer days of cold symptoms but only in the last 2 months of the 4-month trial; the number of colds was not reduced.27
The final study reported that healthy adults at risk who took COLD-fX experienced milder symptoms (Jackson score 77.5 vs 112.3, P
=.002) for fewer days (10.8 vs 16.5 days, P
Only 1 trial has studied P ginseng
; 227 vaccinated healthy adults took 100 mg of an extract or placebo for 12 weeks. Those in the group taking ginseng suffered far fewer colds (15 vs 42, relative risk 0.35, P
Safety and use
: Reported side effects include headache, gastrointestinal upset, anxiety, and insomnia. Ginseng should be avoided during pregnancy and lactation because of potential teratogenicity and estrogenic effects.52
Case reports have described potential herb-drug interactions with phenelzine (induction of mania from depression), warfarin (increased international normalized ratio), and alcohol (increased blood clearance).53
There is conflicting and unclear evidence that P quinquefolius prevents colds. Although it is chemically similar to P ginseng, 1 clinical trial of P ginseng yielded better results, and its long history of traditional use is reassuring. Larger ginseng trials are warranted.
Albert Szent-Györgyi won the Nobel Prize for his discovery of vitamin C. Another Nobel laureate, Linus Pauling, popularized its use for disease prevention and longevity.54
Vitamin C has antioxidant properties, regenerates glutathione, and might stimulate neutrophil and monocyte activity.55
A Cochrane systematic review identified 7 treatment trials evaluating 3294 cold episodes.25
One trial found that patients who took 8 g of vitamin C at the onset of symptoms had more “short colds” of less than a day than those who took 4 g (46% vs 39%, P
=.046). All other trials found no benefit, even at similar doses.25
Prevention: The same review identified 30 prevention trials involving 11 350 subjects. Overall, there was a very slight decrease in the number of colds (odds ratio [OR] 0.96, 95% confidence interval [CI] 0.92 to 1.0) but not in cold severity. Symptom duration decreased by 8.0% in trials that used more than 1 g daily in adults, and decreased by 18% (95% CI 7% to 30%) when this dose was used in children. Assuming that the average cold lasts for 7 to 10 days, this represents 1.5 to 2.0 days shorter duration, which is clinically relevant.
More important, the reviewers identified a subgroup of 6 trials of 642 subjects exposed to severe stress in the form of subarctic cold or intense physical activity. In these trials, those taking vitamin C at doses of 200 to 2000 mg daily had half as many colds as those taking placebo (OR 0.50, 95% CI 0.38 to 0.66). This seems very relevant to patients in many Canadian communities.
Safety and use: Vitamin C is considered safe in doses up to several grams per day. The only occasional side effect is gastrointestinal upset, and doses in excess of 10 g can cause diarrhea. No drug interactions are known.
Taking at least 1 g of vitamin C per day can be recommended for the prevention of colds based on good evidence of moderate reduction in symptom duration in adults and children. While it only reduces symptoms by 1 to 2 days, it is cheap, safe, and simple to use. It can also be recommended for use by athletes in intense training. Further research should explore the potential benefit in Canadians, who are subjected to cold winters.
Allicin is an organosulfur compound found in garlic (Allium sativum
), a food with known cardiovascular56,57
and anticancer benefits.58
Allicin is released when garlic is chopped or chewed, but is inactivated by cooking.59
It has demonstrated antiviral properties in vitro against rhinovirus and several other strains.60
Treatment: There are no trials evaluating allicin for treatment of the common cold.
One study of 146 healthy adults compared a high-dose allicin extract (180 mg daily) with placebo for 12 weeks during the winter months. The results were dramatic; the treatment group had 64% fewer colds (24 vs 65, P
< .001), and symptom duration was reduced by 70% (1.52 vs 5.01 days, P
< .001). Those in the treatment group were much less likely to develop more than 1 cold (2 vs 16 participants developed more than 1 cold).30
Safety and use: It should be emphasized that the allicin preparation used in this trial is unlike typical garlic preparations. It contained 180 mg of allicin; fresh garlic contains 5 to 9 mg per clove, and most extracts contain less than this. The only side-effect reported in the trial was malodorous belching. Little is known about the safety of high-dose allicin, but its use could be considered.
The health benefits of fermented milk were first proposed in the early 20th century. These benefits are due to the presence of probiotic bacteria, which appear to be useful in several gastrointestinal and immune-mediated disorders.61
They interfere with toxin and cell binding sites, and improve mucosal barrier function, intestinal microflora, and gut-associated lymphoid tissue.62,63
Treatment: There are no trials evaluating probiotics for treatment of the common cold.
Probiotics were evaluated in 6 randomized controlled trials31–36
involving 1766 healthy adults. The number of colds was significantly reduced in only 1 of 6 trials,36
symptom severity in 1 of 6,31
and duration in 1 of 5 (P
Two of the negative trials involved atypical populations (military cadets33
and marathon runners32
), but even when these are excluded, the results are unimpressive. There was no clear relationship between response and dose (measured in colony-forming units) or strain in studies that were large enough to be adequately powered. In a recent systematic review, there appeared to be more benefit in children,64
but this is beyond the scope of this review.
Despite their lack of efficacy and potential for harm, antibiotics are still prescribed to patients with the common cold. These drugs alter intestinal flora, which can lead to potentially harmful side effects and complications. We recommend that patients use probiotics whenever antibiotics are prescribed.
Zinc is an essential mineral used in hundreds of biochemical pathways, and deficiency has long been associated with infection risk. Postulated mechanisms in the common cold include interfering with rhinovirus protein cleavage or capsid binding to intracellular adhesion molecule–1 in nasal epithelium,65
and protecting plasma membranes from microbial toxins and complement.55
Zinc was evaluated in 13 trials for treatment of the common cold in adults.16–24,66–69
A nasal spray was compared with placebo in 4 trials,16,67–69
and it reduced symptom duration and severity in only 2 of these.68,69
Irritation by the nasal sprays limits their use; they also appear to yield lower concentrations in the nasopharynx.70
Therefore, they are considered separately.
Zinc lozenges reduced symptom duration and severity in 516,18–21
of 9 trials.16–24
The results are difficult to interpret for 2 reasons. The first is dose; 4 of 5 studies16,18–20
that used higher doses of elemental zinc (13 to 23 mg per lozenge every 2 hours) found a 1.3- to 6.9-day reduction in symptom duration and reduced symptom severity. Negative trials using lower doses have been criticized.71
The second issue is bioavailability. Negative trials have been criticized for using formulations that included citric or tartaric acids,70
sorbitol, or mannitol,72
all of which bind to and inactivate elemental zinc. More positive trials used acetate or gluconate, which do not bind to zinc as tightly as orotate or citrate. The importance of these differences is unclear. These factors might explain the variable results seen in a meta-analysis of 8 randomized controlled trials (OR 0.52, 95% CI 0.25 to 1.2) for symptoms after 7 days.71
It appears that zinc might be effective for treatment of colds, but issues surrounding dose and bioavailability require clarification. Future studies should use higher doses and ensure that formulations do not contain agents that could bind to or otherwise interfere with elemental zinc.
Prevention: There are no trials evaluating zinc for prevention of the common cold.
Safety and use:
Lozenges containing at least 13 mg of elemental zinc can be used every 2 hours at the first sign of a cold. Reported side effects include bitter taste, nausea, and decreased smell and taste.73,74
Prolonged use of zinc (6 to 8 weeks) is not recommended, as it can lead to copper deficiency.