Abstract: Twenty-four double-blind trials with 4631 participants including a total of 33 comparisons of Echinacea preparations and placebo met the inclusion criteria. A variety of different Echinacea preparations based on different species and parts of plant were used. Evidence from seven trials was available for preparations based on the aerial parts of Echinacea purpurea. Ten trials were considered to have a low risk of bias, six to have an unclear risk of bias and eight to have a high risk of bias. Ten trials with 13 comparisons investigated prevention and 15 trials with 20 comparisons investigated treatment of colds (one trial addressed both prevention and treatment). Due to the strong clinical heterogeneity of the studies we refrained from pooling for the main analysis. None of the 12 prevention comparisons reporting the number of patients with at least one cold episode found a statistically significant difference. However a post hoc pooling of their results, suggests a relative risk reduction of 10% to 20%. Of the six treatment trials reporting data on the duration of colds, only two showed a significant effect of Echinacea over placebo. The number of patients dropping out or reporting adverse effects did not differ significantly between treatment and control groups in prevention and treatment trials. However, in prevention trials there was a trend towards a larger number of patients dropping out due to adverse events in the treatment groups. Echinacea products have not here been shown to provide benefits for treating colds, although, it is possible there is a weak benefit from some Echinacea products: the results of individual prophylaxis trials consistently show positive (if non-significant) trends, although potential effects are of questionable clinical relevance.

Plain-language summary: We reviewed 24 controlled clinical trials with 4631 participants investigating the effectiveness of several different Echinacea preparations for preventing and treating common colds or induced rhinovirus infections. Our review shows that a variety of products prepared from different Echinacea species, different plant parts and in a different form have been compared to placebo in randomized trials. Due to the significant differences in the preparations tested, it was difficult to draw strong conclusions. Five trials were rated as having a low risk of bias in all five categories of the Cochrane 'Risk of bias' tool. Five more trials were rated as low risk of bias, having an unclear risk of bias in only one category. Eight trials were rated as having a high risk of bias in at least one category and the remaining six as having an unclear risk of bias. The majority of trials investigated whether taking Echinacea preparations after the onset of cold symptoms shortens the duration, compared with placebo. Although it seems possible that some Echinacea products are more effective than a placebo for treating colds, the overall evidence for clinically relevant treatment effects is weak. In general, trials investigating Echinacea for preventing colds did not show statistically significant reductions in illness occurrence. However, nearly all prevention trials pointed in the direction of small preventive effects. The number of patients dropping out or reporting adverse effects did not differ significantly between treatment and control groups in prevention and treatment trials. However, in prevention trials there was a trend towards a larger number of patients dropping out due to adverse events in the treatment groups. The evidence is current to July 2013.