Review: Insufficient evidence exists on the efficacy of zinc lozenges for cold symptoms
ACP J Club. 1999 Nov-Dec;131:69. doi:10.7326/ACPJC-1999-131-3-069
Marshall I. Zinc for the common cold. Cochrane Review, latest version 15 Feb 1999. In: The Cochrane Library. Oxford: Update Software.
Are zinc lozenges efficacious in reducing the incidence, severity, and duration of cold symptoms?
Studies were identified by searching MEDLINE (1966 to 1997), EMBASE/Excerpta Medica (1974 to 1997), and the Cochrane Library. Bibliographies of relevant papers were searched to identify unpublished trials.
Randomized, double-blind, placebo-controlled trials were selected if they evaluated interventions that began within 3 days of development of cold symptoms and compared treatment with zinc or placebo lozenges taken every 1.5 to 2 hours during waking hours for more than 6 h/d for a duration of ≥ 5 consecutive days. Outcomes had to include incidence and severity of throat and nasal symptoms and cough.
Data were extracted on study methods, participants, intervention, and outcomes. Methodologic quality of studies (selection, performance, exclusion, and detection bias) was assessed by 2 reviewers using 11 criteria; each study was rated as having high, medium, or low quality.
7 trials (n = 754) met the inclusion criteria. All were rated as having medium-to-high methodologic quality. 5 trials involved participants with community-acquired infections, and 2 trials involved volunteers who were experimentally inoculated with human rhinovirus. Data for most outcomes could not be pooled because of study differences. Pooled analyses were based on intention-to-treat results and used a fixed-effects model. After 3 days of treatment, no difference was found between zinc and placebo for the proportion of patients with symptoms (3 studies, n = 419) (Table). The results for symptomatic patients after 5 days (3 studies) and after 7 days (5 studies) of treatment were heterogeneous, and therefore the pooled results were deemed inappropriate. However, pooled results for 2 of 5 studies (n = 187) did show that fewer patients in the zinc group had cold symptoms after 7 days of treatment (Table).
Insufficient evidence exists to support the efficacy of zinc lozenges for reduction of the incidence, severity, and duration of cold symptoms.
Source of funding: No external funding.
For correspondence: Mr. I. Marshall, National Center of Epidemiology and Population Health, Australian National University, Canberra, Australia, 0200. FAX 61-2-62490740.
Table. Zinc vs placebo for cold symptoms*
|Treatment duration||Weighted event rates||RRR (95% CI)||NNT (CI)|
|3 days||72.0%||72.6%||1% (-11 to 12)||Not significant|
|7 days||14.9%||48.9%||69% (48 to 82)||3 (3 to 5)|
*Abbreviations defined in Glossary; RRR, NNT, and CI calculated from data in article.
Evidence of the efficacy of treatments for the common cold are important, given the morbidity and costs to society resulting from coryzal illness. Zinc has antiviral properties in vitro, and following the report of the study of Eby and colleagues (1) in 1984, zinc lozenges have been proposed for the treatment of colds. Since this early trial, several other trials have reported mixed results. The systematic review by Marshall provides an important summary of the evidence from placebo-controlled trials of zinc lozenges.
The main difficulty in summarizing the evidence is the heterogeneity of the data, which limits the ability to pool the results from different studies. Possible sources of heterogeneity are the different viruses, the formulation and dose of zinc, the difficulty of blinding patients to their allocation to treatment or placebo (because of the side effects of zinc), the setting, and the duration of illness before treatment. There is no evidence of benefit after 3 days of treatment and some evidence that fewer patients in the zinc group had cold symptoms at 7 days, but this is based on only 2 studies. Furthermore, the utility of the benefit is questionable because symptoms are usually much less severe at 7 days and any benefit must be balanced against the side effects of zinc. Before the use of zinc lozenges can be recommended in practice, we need more evidence from large randomized controlled trials and a better understanding of the issues surrounding the heterogeneity of the results.
Paul Little, MBBS
Aldermoor Health CentreSouthampton, England, UK