Eighteen RCTs (n=1,650 patients) were included in meta-analyses. The quality of trials was generally good, with eleven trials scoring at least 4 points.
Probiotics were associated with a significant reduction in the rate of irritable bowel syndrome persisting symptoms (RR 0.71, 95% CI 0.57 to 0.88; NNT 4, 95% CI 3 to 12.5; ten RCTs), a significant improvement of irritable bowel syndrome symptoms (SMD -0.34, 95% CI -0.60 to -0.07; 15 RCTs), and a significant improvement of pain scores (SMD -0.51, 95% CI -0.91 to -0.09; ten RCTs) compared with placebo. Significant heterogeneity was observed in all these outcomes. When one outlying trial was removed for both outcomes of improvement of symptoms as a continuous variable and pain scores, significant results for the treatment effect remained but heterogeneity was no longer statistically significant.
Sensitivity analyses did not materially alter the results for the continuous outcome of improvement of symptoms. However, for trials that reported a dichotomous outcome, trials with a Jadad score of at least 4 had a significantly less treatment effect than those with a Jadad score of less than 4 (p=0.02).
Evidence of publication bias was only observed for the outcome of rate of persisting symptoms. Results of other individual symptoms such as bloating, flatulence and urgency were also reported, as were results of subgroup analyses.
Six trials reported that there were no adverse events in the probiotic or the placebo groups. For three trials reporting overall adverse events, there was no significant difference in the rate of adverse events between the probiotic and placebo groups.