Probiotics in Irritable Bowel Syndrome: An Umbrella Review of 27 Systematic Reviews on Methodological Quality and Certainty of Evidence

Background/Objectives: Irritable bowel syndrome (IBS) is a common and heterogeneous gastrointestinal disorder. Although numerous systematic reviews (SRs) have evaluated the effects of probiotics in IBS, uncertainty persists regarding their clinical effectiveness, methodological quality, and certainty of evidence. This umbrella review aimed to critically appraise SRs on probiotics in IBS, quantify overlap among reviews, and assess the certainty of evidence using the GRADE approach. Methods: We conducted an umbrella review of SRs of randomized controlled trials evaluating probiotics in adults with IBS. Searches were performed in MEDLINE/PubMed, Embase, the Cochrane Database of Systematic Reviews, Scopus, and Web of Science from inception to September 2025. Overlap between reviews was assessed using the corrected covered area (CCA). Methodological quality was evaluated with AMSTAR-2, risk of bias with ROBIS, and certainty of evidence with GRADE. Results: Twenty-seven SRs published between 2009 and 2025 were included, encompassing 5–82 randomized trials and 243–10,332 participants per review. Methodological quality was low or critically low across all SRs, with 66.7% rated as critically low by AMSTAR-2 and 85.2% judged at high risk of bias by ROBIS. A high degree of overlap was observed between reviews (CCA: 12%). Probiotics were associated with modest improvements in symptom persistence (risk ratio ≈ 0.78–0.79; number needed to treat 4–7), small-to-moderate effects on abdominal pain (standardized mean difference −0.31 to −0.94) with substantial heterogeneity, and small or inconsistent effects on bloating and quality of life. Adverse events were comparable to placebo. Overall, certainty of evidence was predominantly low or very low, with only 1% of outcomes rated as high certainty. Conclusions: Although probiotics demonstrate statistically significant benefits for some IBS outcomes, the certainty of evidence remains predominantly low or very low due to methodological limitations, inconsistency, imprecision, and substantial overlap between reviews. The accumulation of redundant SRs has not increased confidence in effect estimates. Future efforts should prioritize well-designed, standardized primary trials rather than additional systematic reviews.