## 4 Discussion  
### Probiotics as a Treatment Option for Halitosis  
This systematic review studied probiotics as another treatment option for halitosis, based on the evaluation of existing randomized clinical trials (RCTs). Each of the incorporated studies noted a decrease in halitosis parameters, leading to the conclusion that probiotics can effectively mitigate halitosis. The oral probiotics _W. cibaria_, _L. salivarius_, _S. salivarius_, _B. lactis_, _S. thermophilus_, _L. bulgarcius_, and _L. reuteri_ all demonstrated the ability to decrease VSC levels. In the study conducted by [PERSON] et al., tablets containing _W. cibaria_ led to a significant decrease in overall VSCs and the combined levels of hydrogen sulfide and methyl mercaptan by week 8, which could be because of the increase in the presence of _W. cibaria_ in the oral cavity. The authors explain that _W. cibaria_ produces higher levels of hydrogen peroxide, a potent antibacterial substance, able to modify the composition of bacteria in the oral cavity and hence impeding the growth of _F. nucleatum_, resulting in a significant reduction in the proportion of _F. nucleatum_ in the oral cavity, hence leading to the elimination of harmful bacteria and, consequently, preventing the production of VSCs. Moreover, the authors state that future research is necessary to evaluate whether _W. cibaria_ has a significant inhibitory effect on bacteria producing VSCs by also measuring the bacterial quantity, something that was not performed in their study [20]. This is coherent with the findings of a study carried out by [PERSON] et al., in which they found out that the oral consumption of _W. cibaria_ resulted in lower VSC levels compared to levels observed after taking the placebo [22]. [PERSON] et al. were able to show a decrease in halitosis measured by a Halimeter after the administration of _L. salivarius_ for 10 days in the form of gum. _L. salivarius_ can alter the oral environment by influencing both pH and oxidation-reduction potential. This undermines the stability of pathogens through its antimicrobial properties, reduces the invasion of resident pathogenic bacteria, and stimulates protective immune responses, consequently, effectively hindering the release of VSCs [29]. [PERSON] et al. and [PERSON] et al. observed a similar VSC-decreasing potential of lactobacilli, namely, _L. reuteri_ and _L. salivarius_[26; 28]. [PERSON] et al. detected a significant reduction in methyl mercaptan, a compound closely associated with oral malador originating from periodontal disease, upon the administration of _L. salivarius_[28]. _S. salivarius_ also seems to be able to reduce VSC levels, as [PERSON] et al. and [PERSON] et al. found out in their clinical trials [21; 30]. In the trial carried out by [PERSON] et al., a notable decrease in VSCs was observed while participants were undergoing administration of the probiotic _S. salivarius_; nonetheless, this reduction was not sustained after the course concluded. Along these lines, the study did not recommend relying solely on probiotics as an approach to address persistent tongue-coating halitosis [21]. [PERSON] et al. examined the impact of _S. salivarius_ on oral hygiene indices and halitosis in individuals with orthodontic braces and performed a 3-month follow-up. The authors observed that even though the VSC scores showed a notable decrease in both the probiotic and placebo-control groups  
Figure 6: Forest plot and funnel plot of the effectiveness of probiotics on halitosis in comparison with the placebo group. Values of organoleptic scores at 4 weeks. after the 1-month intervention, during the 3-month intervention-free follow-up, the VSC levels in the placebo group reverted to a value similar to the baseline, whereas in the probiotic group, the VSC levels continued to decrease further from the baseline.