Seventeen trials (2,121 patients, range 12 to 281) were included in the review. All the trials were low or unclear risk of bias in randomisation, allocation concealment and blinding. Some trials were high risk in incomplete outcome assessment, selective reporting and other bias.
Duration of symptoms: The meta-analysis suggested that zinc reduced the duration of cold symptoms (MD −1.65 days, 95% CI −2.50 to −0.81; eight trials; Ι²=95%). Though, there was significant heterogeneity, the quality of the evidence for this finding was considered moderate. Subgroup analysis showed zinc shortened the duration of cold symptoms in adults (MD −2.63, 95% CI −3.69 to −1.58; five studies; Ι²=82%) but not in children (MD −0.26, 95% CI −0.78 to 0.25; three trials; Ι²=84%), greater reduction with high doses of ionic zinc (MD −2.75, 95% CI −3.89 to −1.60; three trials; Ι²=78%) than with lower doses (MD −0.84, 95% CI −1.50 to −0.18; five trials; Ι²=89%).
Severity of symptoms: There was no significant difference in severity of symptoms between the zinc group and the placebo group. The quality of evidence was low given the substantial heterogeneity (Ι²=55%) and imprecision in the summary estimate.
Presence of symptoms at three and seven days: There was no significant difference in the proportion of patients who were symptomatic after three days, but there was a significant difference in the proportion who were symptomatic after seven days, favouring the zinc group (RR 0.63, 95% CI 0.44 to 0.90; nine trials). There was a high level of heterogeneity and the quality of the evidence was considered low.
Adverse events: The occurrence of any adverse event (RR 1.24, 95% CI 1.05 to 1.46, nine trials; Ι²=37%), bad taste (RR 1.65, 95% CI 1.27 to 2.16; eight trials) and nausea (RR 1.64, 95% CI 1.19 to 2.27; nine trials) were more common in the zinc group than in the placebo group. There was no difference between groups in the occurrence of abdominal pain, constipation or diarrhoea.
Other subgroup and sensitivity analysis results were reported.