## 3 Results
### Study Characteristics  
All the clinical trials included systemically healthy subjects. [PERSON] et al. also accepted subjects with gingivitis or mild periodontitis [21]; [PERSON] et al. investigated subjects presenting periodontitis stage III and IV, grade B and C [26]; and [PERSON] et al. performed their study on subjects with chronic periodontitis [24]. The following probiotics were administered: _L. salivarius_[24; 26; 27; 28; 29], _S. salivarius_[21; 30], _L. paracasei_[27], _L. plantarum_[27], _W. cibaria_[20; 22; 31; 32], _S. dentisani_[33], _B. lactis_[23; 34], _L. rhamnus_[35], _L. acidophilus_[26; 36], _B. bifidum_[36], _S. thermophilus_[23], _L. bulgarcius_[23], and _L. reuteri_[24; 26]. Treatment duration ranged from 10 days [29] to 2 weeks [28; 34], 3 weeks [36], 1 month [21; 23; 24; 27; 30; 33; 35], 2 months [20; 22; 31; 32], and 3 months [26]. Most of the studies did not perform any other treatments, except for five studies. [PERSON] et al. administered probiotics in combination with inulin, a prebiotic, to study their effect on halitosis and tongue coating [29]. [PERSON] et al. investigated the effect of oral probiotics on oral hygiene indices and halitosis in patients wearing fixed orthodontic appliances [30]. [PERSON] et al. instructed some of their subjects to use a tongue scraper [23]. In both the study carried out by [PERSON] et al. and by [PERSON] et al., periodontal therapy was performed on all the participants at baseline [24; 26]. Figure 2: Risk of bias assessment. In most of the studies [21; 22; 23; 24; 28; 29; 32; 33], the patients received oral hygiene instructions that mainly encompassed to refrain from other commercial probiotic products and antimicrobial mouth rinses, to maintain their usual oral hygiene and dietary habits, to use toothbrush and toothpaste provided by the investigator, and to abstain from consuming food or beverages, as well as to avoid any oral hygiene practices on the morning of each visit [32]. The outcomes investigated varied depending on the aim of the study; 10 [20; 21; 22; 23; 24; 26; 28; 29; 30; 32; 26; 27; 28; 30; 31; 32] out of the 16 included RCTs investigated the effect of probiotics on oral halitisosis and recorded parameters such as volatile sulfur compound (VSC) levels [20; 21; 22; 23; 26; 28; 29; 30], bad breath improvement (BBI) scores [20; 22], organoleptic scores (OLS) [21; 22; 24; 26; 28; 29], tongue coating scores (TCS) [21; 28; 29; 28], quality of life (QOL) [20; 29; 32], plaque index (PI) [21; 24; 26; 28; 30], gingival index (GI) [30] and modified GI (MGI) [24], probing depth (PD) [21; 24; 26; 28] and probing depth reduction (PDR) [24], bleeding index (BI) [21; 24] and gingival BI (GBI) [26], bleeding on probing (BOP) [21; 26; 28], clinical attachment levels (CALs) [24; 26] and clinical attachment gain (CAG) [24], plaque samples [30], metagenomic analysis [30], Winkel scores [23], Wood's light scores [23], and N-benzoyl-DL-arginine-naphthyl (BANA) scores [24]. In Table 1, a detailed description of these 10 studies investigating the effect of probiotics on oral halitsosis is demonstrated, including authors and year of publication, study design, sample size, health status, probiotic and its form of administration, treatment duration, whether the participants received any other treatment, whether they were given oral hygiene instructions, outcomes investigated, and the study's key findings. The other six studies assessed the clinical efficacy of probiotics on oral health, more precisely. [PERSON] et al. studied the effect of post- and probiotics on oral health parameters such as oral microbiota and oral pathogens, immunological (IgA levels), and plaque levels [27]; [PERSON] et al. assessed whether there is caries activity after the ingestion of probiotics [31]; [PERSON] et al. investigated oral health parameters such as plaque accumulation and saliva quality after the administration of probiotics [33]; [PERSON] et al. evaluated whether probiotic yogart has an effect on initial stages of caries [34]; [PERSON] et al.