The effect of high dose zinc acetate lozenges on the duration of the common cold. In the forest plot on the right side, the vertical line indicates the placebo level. The horizontal lines indicate the 95% CI for the zinc effect and the squares in the middle of the horizontal lines indicate the point estimate of the effect in the particular trial. The sizes of the squares indicate the relative weights of the trials. The diamond shape indicates the pooled effect and the 95% CI. The pooled effect was −42% (95% CI: −35% to −48%; P = 10−33). The duration of colds was transformed to the relative scale so that the duration in the respective placebo group was given the value of 100%. Thus the difference between zinc and placebo groups directly indicates the effect of zinc lozenges in percentages. See Additional file 2 for the extraction of data and for the calculation of the relative mean and SD values for total common cold duration.