Busy individuals with higher levels of psychological stress might be more likely to try zinc to reduce the symptoms of the common cold. However, the efficacy of this practice remains unclear. The current systematic review and meta-analysis by Science and colleagues describes the efficacy and tolerability of zinc for the common cold, and it provides an account of the heterogeneity of results on this topic.
Study Synopsis and Perspective
Oral zinc formulations may lessen the duration of cold symptoms in adults, but not in children, according to the findings of a meta-analysis. Michelle Science, MD, from the Hospital for Sick Children, Toronto, Ontario, Canada, and colleagues published their findings online May 7 in the Canadian Medical Association Journal.
Of the 17 included trials, 8 trials involving patients with naturally acquired colds revealed that zinc reduced the duration of cold symptoms compared with placebo (mean difference, -1.65 days; 95% confidence interval [CI], -2.50 to -0.81 days), albeit with significant heterogeneity (I2, 95%). Subgroup analysis revealed a statistically significant interaction between adults and children (P < .0001), as zinc reduced the duration of cold symptoms in adults (mean difference, -2.63; 95% CI, -3.69 to -1.58), but not in children (mean difference, -0.26; 95% CI, -0.78 to 0.25), with slightly lower heterogeneity (adults, I2, 82%; children, I2, 84%).
The authors noted that a previous meta-analysis reported the efficacy of zinc against common cold symptoms, but that significant heterogeneity was reported for the primary outcome. "The efficacy of zinc therefore remains uncertain, because it is unknown whether the variability among studies was due to methodologic diversity (i.e., risk of bias and therefore uncertainty in zinc's efficacy) or differences in study populations or interventions (i.e., zinc dose and formulation)," the authors write.
After a search of the MEDLINE, Embase, Cochrane, CINAHL, and AMED databases, the authors included randomized controlled trials that evaluated the efficacy of oral zinc as a single agent against placebo or no treatment with no restrictions placed on participant age, language, or year of publication. The authors excluded trials that evaluated intranasally administered zinc or orally administered zinc in combination with other modalities.
Additional subgroup analysis revealed a significant interaction for zinc formulation (P = .003), as zinc acetate (mean difference, -2.67; 95% CI, -3.96 to -1.38), but not zinc gluconate (mean difference, -1.72; 95% CI, -3.89 to 0.44) or zinc sulfate (mean difference, -0.31; 95% CI, -0.89 to 0.28), reduced the duration of symptoms compared with placebo. Regarding adverse events, only bad taste (8 trials; risk ratio [RR], 1.65; 95% CI, 1.27 - 2.16) and nausea (9 trials; RR, 1.64; 95% CI, 1.19 - 2.27) were more frequently observed in patients treated with zinc.
The limitations of the study included the large level of unexplained heterogeneity, the potentially ineffective blinding related to the taste of the placebo, the funding of all included studies by industry, and the performance of most studies in developed countries.
The authors indicated that uncertainty remains regarding the efficacy of zinc against common cold symptoms. "Although oral zinc treatment may attenuate the symptoms of the common cold, large high-quality trials enrolling adults and children are needed," the authors write. "Until further evidence becomes available, there is only a weak rationale for physicians to recommend zinc for the treatment of the common cold."
