Do probiotics help with irritable bowel syndrome itself, or only when taken alongside antibiotics?

Probiotics do genuinely help with irritable bowel syndrome on their own, not just as a supplement to antibiotics. In a large meta-analysis of 82 randomised trials (more than 10,000 patients), probiotics reduced overall irritable bowel symptoms compared with a placebo. Bifidobacterium and Lactobacillus strains also showed a statistically robust benefit in a separate analysis of 54 studies: the chance of symptom improvement was roughly twice as high as with placebo.

Which strain you choose matters a great deal. Bacillus coagulans scored best for abdominal pain, bloating and straining in a network meta-analysis of 43 studies, with results already apparent after 8 weeks of use. At the strain level, Bifidobacterium bifidum (1 billion bacteria per day for 4 weeks) produced the best results in a separate review. A combination of two Lactobacillus strains with one Bifidobacterium and one Streptococcus, also taken for 4 weeks, appeared to perform best among combination products. Notably, the number of bacteria in the capsule made no significant difference across the studies.

How certain is all of this? The statistics point in the same direction, but for most individual strains the quality of evidence is low to very low, and the clinical significance of the improvements found is not always clear. Prebiotics (fibres that feed gut bacteria) and synbiotics (a combination of prebiotics and probiotics) showed no significant improvement in the same network meta-analysis. That is striking, because many people expect that combination to work better.

Probiotics appear to be safe: across 55 studies involving more than 7,000 patients, the risk of side effects did not increase compared with placebo. If you want to combine them with a low-FODMAP diet, which is a popular diet that restricts fermentable carbohydrates, there is a reason to do so: that diet also reduces beneficial gut bacteria, and probiotics are being investigated as a way to compensate for this. The evidence for this is still limited, but it gives you no reason to avoid them.