The duration of the specific symptom as a proportion of the total common cold duration (set as 100%) of the placebo groups. The left side of the figure shows the duration of the symptom as a proportion of the total common cold duration, and the right side shows the same information as a forest plot. On the scale of this figure, 100% corresponds to the total duration of the common cold of the placebo groups. The duration of symptoms was calculated as follows. First, the duration of the specific symptom was calculated as the percentage of the total common cold duration for the placebo group (5.1 days in Petrus et al. , 8.1 days in Prasad et al. (2000) , and 7.1 days in Prasad et al. (2008) ). Then the relative duration of each specific symptom was pooled using the RevMan program. There was no heterogeneity between the three trials in the relative duration of untreated nasal discharge, sneezing, sore throat, hoarseness, muscle pain, headache, and fever with P > 0.05 for the test of heterogeneity. The relative durations of untreated nasal congestion (I2 = 85%), scratchy throat (I2 = 74%), and cough (I2 = 68%) were significantly different in the three trials. See Additional files 2 and 4 for the calculations.