¹No serious study limitations: all the studies had adequately concealed allocation and blinded both participants and study staff to be considered at low risk of bias. Whether free of other bias was unclear in Macknin 1998; Petrus 1998; Turner 2000a; Turner 2000b; Turner 2000c. Petrus 1998 did not adequately describe the sequence generation. Blinding was inadequate in Turner 2000a; Turner 2000b; Turner 2000c.

²Serious inconsistency: there was high statistical heterogeneity. I² statistic = 89%. The heterogeneity was due to differences in the nature of the different interventions (zinc gluconate versus acetate lozenges, zinc lozenges versus zinc syrup), wide dose ranges, varied duration of symptoms prior to administration of zinc (varying from 24 to 48 hours) and characteristics of the study population (children versus adults).

³No serious indirectness: studies both from low‐income and high‐income regions have assessed this comparison. Therefore, the result can be confidently generalised to all situations.

⁴No serious imprecision: though the 95% CI around the pooled effect is narrow, the lower limit does not suggest a clinically important reduction in the duration of cold (a decrease in duration of ≤ 1 day is not shown to be important to patients).

⁵Publication bias cannot be ruled out.

⁶No serious study limitation: all the studies had adequately concealed allocation and blinded both participants and study staff to be considered at low risk of bias. Whether free of other bias was unclear and adequate sequence was not generated in one study (Petrus 1998).

⁷Serious imprecision: the 95% CI around the pooled effect is wide, the lower limit is crossing the point of no effect.

⁸Serious inconsistency: there was high statistical heterogeneity. I² statistic = 84%. The heterogeneity may be due to differences in the nature of the different interventions (zinc gluconate or acetate lozenges, zinc sulphate syrup) and dose range (30 to 160 mg/day) as well as mean duration of symptoms prior to administration of zinc (varying from 24 to 48 hours), as well as the characteristics of the study population (children versus adults). However, subgroup analysis was not possible as there were not enough studies for each variable.

⁹Kurugol 2006b is a community‐based intervention including 200 healthy school children and studying the effect of daily administration of 15 mg zinc sulphate syrup over a period of seven months. Vakili 2009 is also a community‐based intervention including 200 healthy school children and studying the effect of daily administration of 10 mg zinc sulfate tablets over a period of seven months.

¹⁰Serious study limitation: though the study by Kurugol 2006b was of high quality, that by Vakili 2009 was of poor methodological quality.

¹¹Serious inconsistency: there is substantial heterogeneity between the two trials: I² statistic for heterogeneity = 88%. Both trials showed a benefit with zinc, however the size of this effect was much larger in Vakili 2009. The heterogeneity was due to differences in the trial methodology and the nature of the interventions.

¹²No serious imprecision: the 95% CI around the pooled effect is narrow. Even the lower limit suggests a clinically important reduction in the incidence rate ratio of cold which is shown to be important to patients.

¹³No serious study limitations: allocation concealment was unclear in two studies, i.e. Smith 1989 and Weismann 1990, though both the studies blinded both participants and study staff.

¹⁴Serious inconsistency: there was high statistical heterogeneity. I² statistic = 75%. The heterogeneity may be due to differences in the nature of the different interventions (zinc gluconate or acetate lozenges) and dose range (30 to 160 mg/day) as well as mean duration of symptoms prior to administration of zinc (varying from 24 to 48 hours, as well as the characteristics of the study population (children versus adults). However, subgroup analysis was not possible as there were not enough studies for each variable.

¹⁵Serious indirectness: only studies from high‐income regions have assessed this comparison. Therefore, the result cannot be generalised to all situations.

¹⁶No serious imprecision: both limits of the 95% CI suggest a clinically important reduction in proportion of participants given the intervention symptomatic after seven days of treatment.

¹⁷Serious inconsistency: there is substantial heterogeneity between the two trials: I² statistic test for heterogeneity = 64%. Both trials showed reduced days of school absence with intervention, however, the size of this effect was much larger in Kurugol 2006b. The heterogeneity was due to differences in the trial methodology and the nature of the interventions.

¹⁸No serious imprecision: though the 95% CI around the pooled effect is narrow, the lower limit does not suggests a clinically important reduction in the duration of school absence (a decrease in duration of ≤ 1 day is not shown to be important to patients).

¹⁹No serious inconsistency: there was no statistical heterogeneity. I² statistic = 0%.

²⁰No serious imprecision: both limits of the 95% CI suggest a clinically important reduction in the rate of antibiotic use with intervention.

²¹No serious study limitations: all the studies had adequately concealed allocation (except Weismann 1990, in which allocation concealment is unclear) and blinded both participants and study staff to be considered at low risk of bias. Whether free of other bias was unclear in Macknin 1998 and Weismann 1990. Weismann 1990 did not adequately describe the sequence generation.

²²No serious inconsistency: there is no statistical heterogeneity. I² statistic = 21%. Both the lozenges and syrup preparation trials were pooled.

²³No serious imprecision: the 95% CI around the pooled effect is narrow. The resulting adverse events from use of zinc are higher and this is significant.