Fourteen RCTs were included (n>1,125, range 24 to 362 where stated). Quality varied: four studies scored a maximum 4 points, but seven scored 2 or less.
Overall improvement: There was a modest but statistically significant improvement in the probiotics group versus placebo after several weeks of treatment, without significant heterogeneity: OR 1.6 (95% CI 1.2 to 2.2, I2=28%; seven RCTs, n=895); SMD 0.23 (95% CI 0.07 to 0.38, I2=0%, six RCTs, n=657). The number needed to treat was approximately one to 21. Analysis of adults-only data did not change the statistical significance of the results. Sensitivity analysis by study quality materially alter the findings. Assessment for publication bias was inconclusive.
Individual symptoms: Dichotomous data showed statistically significant improvement in the probiotics group versus placebo in abdominal pain (OR 2.88, 95% CI 1.84 to 4.50, I2=24%; seven RCTs, n=398). Analysis of adults-only data did not change the statistical significance of the results. Dichotomous data showed statistically significant improvement in the probiotics group versus placebo in flatulence (OR 2.31, 95% CI 1.37 to 3.9, I2=7%) and bloating (OR 1.75, 95% CI 1.03 to 2.96, I2=0%; four RCTS, n=253). However, continuous data (nine, five and four RCTs) showed no statistically significant difference between probiotics and placebo for these outcomes, with high heterogeneity for analyses of abdominal pain (I2=51%) and flatulence (I2=49%). Data on quality of life were unsuitable for pooling: three of four relevant RCTs found no statistically significant difference between probiotics and placebo.
Adverse effects: None of the 14 RCTs that measured this outcome reported statistically significant findings.
Other results were reported in the review.