Thirteen RCTs were included in the review (n=2,121 randomised, 1,971 analysed).
The methodological quality of the included studies was generally poor, with a median quality score of 2.
Using a random-effects model, there was no statistically significant difference between the use of active extracts and placebo for the prevention of exacerbation (3 studies); the RR was 0.66 (95% CI: 0.41, 1.08). The presence of statistical heterogeneity was identified (P<0.001).
There was a statistically significant benefit for the average duration of an exacerbation in favour of treatment with active extracts compared with placebo (3 trials); the weighted mean difference was -3.3 days. Significant statistical heterogeneity was detected between the trials.
There was a statistically significant difference in favour of the bacterial extracts, compared with placebo, in improvement assessed by observers (5 studies; RR 0.57, 95% CI: 0.49, 0.66) and patients (2 studies; RR 0.44, 95% CI: 0.31, 0.61).
Skin itching or cutaneous eruptions and urologic problems were experienced significantly more by those receiving bacterial extracts than those receiving placebo. No other significant adverse effects were found, and no significant difference in hospitalisation was identified.