ABSTRACT Background Allergic rhinitis (AR) is an increasingly common chronic inflammatory condition of the nasal mucosa. While conventional anti‐allergy treatments are widely used, they can come with side effects and are not always effective. As a result, AR remains a significant concern for many millions of people worldwide, affecting quality of life and social functioning. Objective To investigate clinical evidence for the role of probiotics and postbiotics in reducing AR symptoms. Methods In this single‐centre, randomized, double‐blind, placebo‐controlled clinical trial, 72 adults aged 18–60 years with moderate–severe AR, taking first‐line medication, were studied. The primary outcome of the trial was to determine the effects of supplementation with the probiotic Bifidobacterium longum CECT 7347 (PRO) and the heat‐treated postbiotic of the same strain (POST) on symptoms associated with AR as assessed using the Combined Symptom and Medication Score (CSMS). The secondary outcomes of the study assessed the effects of the interventions over: quality of life (rhinitis quality of life questionnaire RQLQ), systemic immune response, local immune response, and faecal microbiota composition. Results Statistically significant reduction in total CSMS after 8 weeks of supplementation was observed in the POST group compared to placebo control (CON) ( p = 0.022), showing on average a 0.6 greater reduction than placebo (CI: 0.09–1.12), equivalent to a 33.15% greater reduction and exceeding the World Allergy Organization's threshold for minimal clinically significant efficacy. Conclusion POST intake for 8 weeks was associated with a statistically significant reduction of total CSMS compared to placebo. There was also improvement in RQLQ parameters when compared to baseline. PRO intake for 8 weeks resulted in non‐significant beneficial effects on AR‐related symptoms. Trial Registration ClinicalTrials.gov Identifier: NCT05208528
Cardoso et al. (2026) studied this question.