<p>Halitosis, or bad breath, is associated with oral microbial imbalances and the production of volatile sulfur compounds (VSCs). While existing treatments target pathogenic bacteria or oral health indicators, they may not address the underlying systemic complexity. This study explored the efficacy of Complex OK oral probiotics containing <i>Lactobacillus gasseri</i> HHuMIN D and <i>L. paracasei</i> OK in mitigating halitosis by evaluating VSC levels and metabolic markers. A 12-week, randomized, double-blind, placebo-controlled clinical trial was conducted involving 80 participants, 70 of whom completed the study in South Korea (KCT0009894). The participants were selected based on the presence of 2 of 3 pathogenic oral bacteria (<i>F. nucleatum</i>, <i>P. gingivalis</i>, and <i>P. intermedia</i>) and baseline VSCs &gt; 2.0&#xa0;ng/10&#xa0;mL. Exclusion criteria included systemic diseases, recent antibiotic/probiotic use, and severe dental conditions. Oral health, VSCs, and metabolic markers were assessed using paired <i>t</i>-tests, ANCOVA, and Wilcoxon rank-sum tests. Despite unchanged oral health indicators and levels of harmful bacteria, probiotic supplementation showed efficacy in maintaining microbial balance. Significant reductions in H₂S and total VSCs were observed in the experimental group compared to the placebo (<i>P</i> &lt; 0.05). No significant changes were observed in oral health indices or levels of harmful oral bacteria, but the experimental group showed a significant decrease in blood glucose (<i>P</i> = 0.009) and an increase in phosphorus levels (<i>P</i> &lt; 0.05). This study provides the first published evidence linking systemic metabolic regulation to halitosis reduction, suggesting that probiotics mitigate bad breath through glucose and phosphorus metabolism rather than by direct bacterial inhibition. Further research is needed to confirm these findings and to explore the underlying mechanisms.</p>