This study aimed to evaluate the clinical effectiveness of the probiotic Lactobacillus reuteri as an adjunct to non-surgical periodontal therapy (NSPT).A double-blind, paralleled-arm, placebo-controlled, randomized clinical trial was conducted. Probiotics L. reuteri or placebo lozenges were randomly prescribed for use twice-daily for 28 days. Primary outcome were clinical attachment levels (CAL) and probing pocket depths (PPD). All participants underwent NSPT and follow-up clinical assessments were performed at Day 90 and Day 180.The trial response rate was 69.5% (41 out of 59). Among the test and control groups, there were significant intra-group differences in primary outcomes: CAL (both, p<0.001) and PPD (both, p<0.001); and in secondary outcomes: percentage of sites with 'bleeding on probing' (both, p<0.001) and visible plaque (both, p<0.001). No statistically significant inter-group differences in any outcomes at any time points (all, p>0.05) nor in the changes in outcomes (∆) with time (all, p>0.05). There was a trend of a greater magnitude of statistical change occurring among the test group compared to the control group.The adjunctive use of probiotics with NSPT did not show any additional clinical effectiveness when compared to NSPT alone in the management of periodontitis (ChiCTR-IOR-17010526). This article is protected by copyright. All rights reserved.