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4 May 2011
A Cochrane review concluded that oral zinc may have benefits for prevention and treatment of the common cold. However, these benefits are uncertain because of marked heterogeneity between the studies included in the analysis. Furthermore, any possible benefits of zinc need to be weighed against the risk of side effects such as nausea.
Level of evidence:
Level 2 (limited quality patient-oriented evidence) according to the SORT criteria.
Action
While zinc supplementation is clearly appropriate for people who are zinc deficient, and who cannot obtain enough zinc from their diet, healthcare professionals should not recommend the use of supplemental zinc for the treatment or prevention of the common cold on the basis of this review. Healthcare professionals should advise patients who are considering taking zinc supplements about the risk of side effects, the limitations of the clinical evidence for benefit and the uncertainty about the most appropriate dose, formulation and duration of treatment. It should also be remembered that zinc is associated with a number of drug interactions, notably a reduction in the absorption of tetracycline and quinolone antibacterials.
What is the background to this?
There is no proven method of prevention or treatment for the common cold. The review considered randomised, double-blind, placebo-controlled trials using zinc for at least five consecutive days to treat, or for at least five months to prevent, the common cold.
What does this study claim?
On the basis of 13 therapeutic trials (n=966) and two preventive trials (n=394) in adults and children, this systematic review found that zinc administered (dose range 10–276mg/day as lozenge, or syrup in two studies) within 24 hours of onset of symptoms, reduced the duration and severity of the common cold in healthy people compared with placebo. Results indicate that, on average, for about every six people treated with zinc for their cold, one would be free of symptoms at 7 days, who would not have been otherwise. Results from the prevention studies, suggested that zinc supplements (as syrup or tablet) taken for at least five months reduced cold incidence, school absenteeism and prescription of antibiotics in children.
Zinc was associated with significantly more adverse effects than placebo, overall, and specifically for nausea and bad taste. The results suggest that, on average, for every 13 people treated with zinc for a common cold, one would suffer an adverse event who would not have done so otherwise. (See below for more detailed study results).
In view of the differences between the studies with regard to study populations, dosages, formulations and duration of treatment, no firm recommendations could be made about the dose, formulation and duration that should be used.
How does this relate to other studies?
This Cochrane review, updated a previous withdrawn review, published in 1999, which found that there was no strong evidence from trials that taking zinc can help improve the common cold.
So what?
Zinc is a trace element that has several important functions in the body, but most people can get enough zinc in their diet. Foods rich in zinc include meat, shellfish, milk, dairy foods, such as cheese, bread, and cereal products, such as wheat germ. Zinc supplementation is clearly appropriate for people who are zinc deficient, and who cannot obtain enough zinc from their diet.
A meta-analysis of randomised controlled trials is usually the appropriate way to pool results from several studies to estimate the overall effectiveness of an intervention. However, where there is a high level of heterogeneity between studies, as was the case in this review, pooling the results to provide an overall level of effectiveness is questionable. It certainly makes the findings less certain. The authors note the high level of heterogeneity is likely to be due to the different zinc preparations and doses used in the studies, the study populations that were combined (adults and children) and differences in the duration of cold symptoms before supplementation began. Furthermore, the infecting agent was not known in most of the trials as most of these relied on community-acquired infections, in which the infecting agent and infection rates were not determined.
It is possible that zinc will benefit some groups of people more than others, such as those with lowered immune systems. However, this review did not find any studies in immunocompromised populations.
Bearing in mind the limitations of the analysis, the uncertainty of the results, and the adverse effects associated with oral zinc, it does not seem appropriate to recommend zinc supplements for the treatment or prevention of the common cold until further research has been carried out.
Design
Systematic review and meta-analysis of randomised, double-blind, placebo-controlled trials
Patients
13 therapeutic trials (n=966) and two preventive trials (n=394) in adults and children (ages 1 to 65 years). All 15 trials were conducted in high-income countries among healthy people
Intervention and comparison
Zinc provided in the form of syrup, lozenges or tablets was compared with placebo
Outcomes and results
Intake of zinc was associated with a significant reduction in the duration (standardised mean difference [SMD] –0.97; 95% confidence interval [CI] –1.56 to –0.38. P=0.001), and severity of common cold symptoms (SMD –0.39; 95% CI –0.77 to –0.02, P=0.04). There was a significant difference between the zinc and control group for the proportion of participants symptomatic after seven days of treatment (OR 0.45; 95% CI 0.2 to 1.00, number needed to treat [NNT 6], P=0.05). There was no significant difference in the proportion of participants who were symptomatic after 3 or 5 days. When supplemented for at least five months, zinc reduced cold incidence, school absenteeism and prescription of antibiotics in children. The risk ratio (RR) of developing a cold (RR 0.64; 95% CI 0.47 to 0.88, P=0.006, NNT 5), school absence (SMD –0.37, 95%CI –0.70, –0.04, P=0.03) and prescription of antibiotics (RR 0.27, 95%CI 0.16 to 0.46, P<0.00001, NNT 5) was lower in the zinc group.
Overall adverse events (number needed to harm (NNH) 13; odds ratio [OR] 1.59; 95% CI 0.97 to 2.58) (P = 0.06), bad taste (OR 2.64; 95%CI 1.91 to 3.64, P<0.00001) and nausea (OR 2.15; 95%CI 1.44 to 3.23, P=0.002) were higher in the zinc group.
Sponsorship
Post Graduate Institute of Medical Education and Research (PGIMER), Chandigarh, India.
More information on common respiratory infections can be found in the NPC Common Infections – Respiratory Tract e-learning materials
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