On 2014 Dec 06, Harri Hemila commented:

Jackson JL, 2000 found statistically highly significant heterogeneity between the zinc lozenge trials (P<0.00001 ). They calculated a pooled estimate of effect, although firm evidence of heterogeneity introduces serious doubt about the relevance of any one overall estimate. Instead, the main focus should be on trying to understand the sources of heterogeneity, Thompson SG, 1994. Although Jackson noted that some of the negative results might have been caused by low zinc availability, they did not carry out subgroup analysis by zinc doses. Jackson concluded in the Discussion that their “meta-analysis suggests that the evidence of zinc effectiveness is still lacking” , which is based on their inappropriate pooling of the low and high dose trials together.

Two further RCTs on zinc acetate lozenges were published after the review and both of them found significant benefit of zinc, Prasad AS, 2000 and Prasad AS, 2008.

Hemilä H, 2011 analyzed the dose-response in the zinc lozenge trials and found that five trials that used the lowest doses of zinc uniformly found no effect. Three trials used zinc acetate in daily doses of over 75 mg, and the pooled result indicated a 42% reduction in the duration of colds (95% CI: 35% to 48%). Five trials used zinc salts other than acetate in daily doses of over 75 mg, the pooled result indicating a 20% reduction in the duration of colds (95% CI: 12% to 28%). Most of the studies had been published before 2000, and the dose response would have been evident then.

On 2014 Dec 06, Harri Hemila commented:

Jackson JL, 2000 found statistically highly significant heterogeneity between the zinc lozenge trials (P<0.00001 ). They calculated a pooled estimate of effect, although firm evidence of heterogeneity introduces serious doubt about the relevance of any one overall estimate. Instead, the main focus should be on trying to understand the sources of heterogeneity, Thompson SG, 1994. Although Jackson noted that some of the negative results might have been caused by low zinc availability, they did not carry out subgroup analysis by zinc doses. Jackson concluded in the Discussion that their “meta-analysis suggests that the evidence of zinc effectiveness is still lacking” , which is based on their inappropriate pooling of the low and high dose trials together.

Two further RCTs on zinc acetate lozenges were published after the review and both of them found significant benefit of zinc, Prasad AS, 2000 and Prasad AS, 2008.

Hemilä H, 2011 analyzed the dose-response in the zinc lozenge trials and found that five trials that used the lowest doses of zinc uniformly found no effect. Three trials used zinc acetate in daily doses of over 75 mg, and the pooled result indicated a 42% reduction in the duration of colds (95% CI: 35% to 48%). Five trials used zinc salts other than acetate in daily doses of over 75 mg, the pooled result indicating a 20% reduction in the duration of colds (95% CI: 12% to 28%). Most of the studies had been published before 2000, and the dose response would have been evident then.