The Efficacy of Probiotics in Treating Upper Respiratory Tract Infections, Allergic Rhinitis, and Chronic...

Background: Upper respiratory tract infections (URTIs), allergic rhinitis (AR), and chronic rhinosinusitis (CRS) are prevalent and burdensome inflammatory disorders. Probiotics may modulate immune responses via gut-respiratory axis signaling, but their clinical efficacy across these conditions remains uncertain and highly heterogeneous. Methods: We conducted a PRISMA-guided systematic review and random-effects meta-analysis of randomized controlled trials (RCTs) evaluating oral or topical probiotics for URTIs, AR, or CRS (MEDLINE, EMBASE, CENTRAL, and Web of Science; inception to July 2025). Disease severity category (acute, subacute, chronic), episode incidence, and duration of illness were extracted alongside symptom scores. Risk of bias was assessed using the Cochrane RoB 2 tool, and certainty of evidence was graded using the GRADE framework. Results: Thirty-two RCTs were included. In URTIs, certain strains [e.g., Lactiplantibacillus plantarum DR7, Lactobacillus rhamnosus GG] reduced symptom duration and antibiotic use; however, the pooled incidence reduction was non-significant (RD = -0.07; 95% CI: -0.23 to 0.09; p = 0.38), with very high heterogeneity (I² = 93.12%), limiting interpretability. In AR, probiotics reduced TNSS and improved quality of life (SMDs -0.72 to -2.30) in individual trials supported by immune marker changes [e.g., increased IL-10, decreased IgE]. In CRS, only two small trials-differing in delivery route (topical vs. oral), CRS phenotype, and publication era (2009 and 2017)-with conflicting effect directions were identified; formal meta-analysis was not performed given insufficient and methodologically heterogeneous data, and CRS findings are reported descriptively only. GRADE certainty ranged from very low (URTI incidence) to low (AR symptoms, URTI illness burden). Conclusions: Probiotic effects appear strain- and condition-specific. URTI pooled incidence data are unreliable due to extreme heterogeneity; individual strains show consistent benefits on illness burden and AR symptoms/quality of life. Evidence for CRS is insufficient for meta-analytic conclusions; findings are reported descriptively pending adequately powered dedicated trials. Strain-targeted RCTs with standardized outcomes, formal GRADE appraisal, and adequate power are needed before clinical recommendations can be made.