Effectiveness of Probiotics in Managing Oral Halitosis: A Systematic Review of Randomized Controlled Trials

ABSTRACT

Aim:

Halitosis, characterized by volatile sulfur compound (VSC) production, is a prevalent oral health concern affecting 31.8% of the global population. Conventional treatments such as chlorhexidine-based mouthwashes offer temporary relief, but probiotics have emerged as a promising biological therapy by modulating oral microbiota. We aimed to systematically evaluate the efficacy of probiotic monotherapy in reducing halitosis-related VSC levels and organoleptic test (OLT) scores in randomized controlled trials (RCTs).

Materials and Methods:

A comprehensive database search (PubMed, Scopus, ClinicalTrials.gov, and gray literature) was conducted following PRISMA guidelines. The review protocol was prospectively registered with the Open Science Framework (OSF). Only RCTs comparing probiotics to placebo in systemically and periodontally healthy adults were included. Primary outcomes were changes in VSC levels and OLT scores, assessed using halimeters, Oral Chroma devices, or OLT evaluation. Risk of bias was assessed using the Cochrane RoB 2 tool.

Results:

Six RCTs (n = 360 participants) met the inclusion criteria. Five studies demonstrated significant VSC reduction after probiotic intervention (P < 0.05). Three studies reported OLT score improvements, particularly with Streptococcus salivarius K12 and Weissella cibaria. Four studies confirmed microbiome alterations, with the effects of probiotics persisting post-treatment. No serious adverse effects were reported.

Conclusion:

Probiotics significantly reduce VSC levels, improve OLT scores, and modulate the growth of oral microbiota. However, the heterogeneity of studies and limited long-term follow-up hinder clinical translation. Larger, standardized trials are essential for establishing the clinical efficacy. Probiotics present a safe adjunct therapy for halitosis management. Future research should focus on longitudinal studies, specific strain efficacy, and microbiome-targeted interventions.

INTRODUCTION

Halitosis, often known as oral malodor or bad breath, is characterized by an unpleasant odor emanating from the oral cavity. This condition can have a significant negative impact on social life and professional relationships, and it represents one of the most common concerns in dental clinics. The global prevalence of halitosis is estimated at approximately 31.8%, with reported rates ranging from 2.4% to 55% across different populations, influenced by regional and socioeconomic variables.[1]

The primary etiology of halitosis is intraoral and is closely linked to the production of volatile sulfur compounds (VSCs), such as hydrogen sulfide (H₂S), methyl mercaptan (CH₃SH), and dimethyl sulfide [(CH₃)₂S]. These substances result from anaerobic bacterial degradation of sulfur-containing proteins in the mouth.[2,3] Factors that contribute to this process include inadequate oral hygiene, accumulation of tongue coating, periodontal conditions, and habits like smoking and alcohol use.[2] The posterior region of the tongue, in particular, provides a favorable environment for bacterial colonization and debris accumulation, both of which are key contributors to VSC formation.[2,3]

Traditional management strategies for halitosis focus on mechanical cleaning, such as tongue brushing or scraping, along with the use of chemical agents like chlorhexidine (CHX).[2] Two prior systematic reviews evaluated the efficacy of mouth rinses on halitosis, regardless of their active components. Both reviews supported the beneficial effects of rinses containing CHX, zinc (Zn), and cetylpyridinium chloride (CPC), although they provided limited analysis of other commercially available products.[4,5]

As interest grows in alternatives to conventional treatments, probiotics have gained attention as a novel option with promising potential. Defined as live microorganisms that confer health benefits to the host when administered in adequate amounts, probiotics may help combat halitosis by inhibiting the growth of odor-producing bacteria, enhancing immune modulation, and secreting antimicrobial compounds.[6] Additionally, probiotics have shown potential as supportive therapies in the management of periodontal diseases and dental caries.[6,7]

Emerging clinical evidence highlights the effectiveness of specific probiotic strains—such as Streptococcus salivarius K12 and Weissella cibaria—in lowering VSC concentrations and promoting oral health. However, many of these studies suffer from methodological limitations, including small sample sizes and variations in probiotic formulations, making it difficult to draw definitive conclusions.[8,9] Moreover, earlier systematic reviews have not comprehensively compared different probiotic strains or evaluated their sustained effects over time, leaving an important gap in the existing literature.

This systematic review seeks to bridge that gap by assessing the effectiveness of probiotics in the management of halitosis, with a focus on comparing different strains and evaluating their long-term impacts. The analysis compares probiotic interventions with placebo controls to contribute to a clearer understanding of their potential role in oral healthcare. The review process was guided by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework.[10]

MATERIALS AND METHODS

FOCUSED QUESTION AND REGISTRATION

The focused question of this review was framed using the established Population, Intervention, Comparison, Outcome, and Studies (PICOS) format: In systemically and periodontally healthy individuals, what is the impact of administering probiotics in any form as monotherapy on intraoral malodor, as measured by VSC levels and/or organoleptic (OLT) scores in randomized controlled clinical trials (RCTs)?

This review was developed and conducted in accordance with a predefined protocol, which was registered and reported following standard methodological guidelines (OSF Registration DOI: https://doi.org/10.17605/OSF.IO/H5UGZ). Since the review did not involve direct research on human participants, ethical approval was not necessary.

STUDY DESIGN

The primary aim of this review was to assess the efficacy of probiotics, in comparison to placebo, in the treatment of intraoral halitosis. For this purpose, RCTs were considered the most appropriate study design. Only studies that evaluated probiotics as a standalone intervention for halitosis of intraoral origin and used placebo as a comparator were included.

INCLUSION CRITERIA

Studies were considered eligible if they met all of the following criteria: (a) conducted in human subjects; (b) participants were systemically healthy and free of active periodontitis; (c) probiotic intervention was specified and used alone; (d) placebo served as the comparator; (e) primary outcomes were based on OLT scores and/or VSC levels; (f) included a sample size greater than 20 participants; and (g) utilized an RCT design.

EXCLUSION CRITERIA

Studies were excluded based on the following criteria: (a) animal or laboratory-based studies; (b) trials lacking a control group; (c) expert opinions; (d) editor’s selections; (e) author or editor responses; (f) interviews; (g) commentaries; (h) book chapters or conference abstracts; (i) article summaries; (j) cross-sectional studies; (k) uncontrolled case series; (l) individual case reports; (m) case-control studies; (n) cohort studies; (o) prospective non-randomized trials; (p) retrospective trials; (q) narrative reviews; (r) systematic reviews; (s) meta-analyses; (t) articles not published in English; (u) studies without reported outcomes; (v) in vitro research; (w) participants with active periodontitis; (x) studies with fewer than 20 subjects; (y) studies involving more than one intervention; (z) those including chlorhexidine, inulin, tongue scraping, or any other disinfection procedure alongside probiotics, or vice versa; (aa) trials without a placebo comparator; (ab) studies not using VSC levels or OLT scores as outcome measures; and (ac) studies where probiotics were not specifically defined (e.g., unspecified probiotic yogurts or solutions).

OUTCOME MEASURES

Primary outcomes were defined as changes in OLT scores and/or VSC levels. Secondary outcomes considered included microbial profile alterations, tongue coating characteristics, and periodontal indices.

SEARCH METHODS FOR STUDY IDENTIFICATION

A systematic search was performed across the following databases: (1) MEDLINE (via PubMed), (2) Scopus, (3) Science Direct, (4) Google Scholar, (5) Livivo, (6) Clinicaltrials.gov (National Library of Medicine), (7) Ovid, (8) greylit.org, and (9) Meta Register of Controlled Trials (WHO). A database-specific search strategy was implemented for each source, derived from the primary strategy created for MEDLINE. Literature up to September 2024 was included in the search [Table 1].

Table 1.

Search strategy for identifying papers investigated the use of probiotics in the management of halitosis

Database	Search strategy used	Limits
Medline (via PubMed)	(halitosis OR malodor OR malodour OR bad breath OR fetor oris) AND probiotics	No
Scopus	(TITLE-ABS-KEY ((halitosis OR malodor OR “bad breath” OR “fetor oris”)) AND TITLE-ABS-KEY (probiotics))	Title, abstract, keywords:
Science Direct	Title, abstract, keywords: (halitosis OR malodor OR “bad breath” OR “fetor oris”) AND probiotics	Title, abstract, keywords:
Google Scholar	allintitle: halitosis probiotics OR malodor OR “bad breath” OR “fetor oris”	Title
Livivo	TI=(halitosis OR malodor OR malodour OR bad breath OR fetor oris) AND TI=probiotics	Title
Clinical Trials.gov	condition or disease:halitosis, other terms: probiotics	No
Ovid	(AllFields: (halitosis OR malodor OR malodour OR “bad breath” OR “fetor oris”)AND (probiotics))	All
greylit.org	halitosis and probiotics	No
meta register of controlled trials (https://trialsearch.who.int/)	halitosis and probiotics	No

SEARCH STRATEGY AND DATA EXTRACTION

Two independent reviewers (G.P. and C.N.) conducted the screening and selection process in multiple stages. Initially, titles and abstracts were reviewed to identify studies that potentially met the eligibility criteria. Full texts of selected articles were then assessed in detail for inclusion. Discrepancies between reviewers were resolved by consulting a third author (K.P.).

Data extraction was also carried out independently by the same reviewers using a structured form. Key data included general study information (e.g., design type, number of subjects, eligibility criteria, outcome measures, and institutional affiliation), specifics of the intervention (probiotic strain, dosage form, and duration of treatment), and reported outcomes (primary results, outcome metrics, clinical findings, and publication availability).

This systematic approach allowed for reliable and consistent data collection between reviewers.

ASSESSMENT OF RISK OF BIAS IN INDIVIDUAL STUDIES

All included RCTs were evaluated for risk of bias using the Cochrane Risk of Bias Tool for Randomized Clinical Trials (RoB 2) (last updated August 2019).[11] Two reviewers (G.P. and C.N.) assessed each study independently and in duplicate. The tool examines five areas: bias in the randomization process, bias due to deviations from intended interventions, bias from missing data, bias in measurement of outcomes, and bias in reporting of results.

Based on these domains, studies were classified as having low risk of bias, some concerns, or high risk. Any disagreements in judgment were resolved through discussion with a third author (S.D.).

RESULTS

STUDY SELECTION

A total of 776 publications were identified through the comprehensive search strategy [Figure 1].[12] After eliminating duplicates, 268 unique records remained. Title and abstract screening led to the exclusion of 241 articles, leaving 27 for full-text evaluation. Of these, 21 were excluded for the following reasons: (a) incorrect study design/non-RCT (n = 5)[6,13,14,15,16]; (b) lack of a placebo control group (n = 2)[17,18]; (c) presence of multiple interventions (n = 9)[8,19,20,21,22,23,24,25,26]; (d) no available results (n = 1)[27]; (e) primary outcomes not relevant (n = 2)[28,29]; (f) unspecified probiotic used (n = 1)[30]; and (g) inclusion of subjects with active periodontitis (n = 1).[31] Ultimately, six studies met the eligibility criteria and were included in this systematic review.[32,33,34,35,36,37]

Figure 1.

Prisma flow diagram depicting the selection process of studies

DESCRIPTION OF THE STUDY POPULATION

An overview of the populations and study designs is provided in Table 2. All included trials involved participants without periodontitis. Of these, three studies[34,36,37] reported on periodontal indices such as PD, GI, PI, CI, and BOP/BI. The other three studies[32,33,35] did not specify periodontal parameters, although all confirmed participants were free of active periodontitis—either due to prior treatment or absence of disease at the baseline.

Table 2.

Characteristics of included studies

Study	Country	Study design	Age range (mean), sex	No of participants	Pretreatment	Definition of halitosis	Vehicle
Keller et al.[32]	Denmark	Double-blinded, placebo-controlled, RCT	19–25 (22.0)	28a/25b	None	OLT>1	Gum
Han et al.[33]	Korea	Double-blinded, placebo-controlled, RCT	20–70 (51.7) 28M, 63F	100a/91b	None	VSC>1.5ng/10 mL	Tablet
Benic et al.[34]	New Zealand	Triple-blinded, placebo-controlled, RCT	10–30 (14.9) 23M, 41F	64a/64b	None	Not stated	Lozenge
Lee et al.[35]	Korea	Double-blinded, placebo-controlled, RCT	>20 (23.5) 43M, 25F	92a/68b	Dental scaling and root planning	VSC>1.5ng/10 mL	Tablet
He et al.[36]	China	Double-blinded, placebo-controlled, RCT	23–44 (30.04) 8M, 20F	33a/28b	Periodontal treatment	OLT score ≥ 2 VSC ≥ 1.5 ppb	Lozenge
Ding et al.[37]	China	Double-blinded, placebo-controlled, RCT	Age range not stated (22.64) 15M, 18F	43a/33b	None	VSC ≥ 200ppb	Powder

Study	Frequency/Intervention period		Administration method	Probiotics (strains and dose)		Outcomes of interest	Follow-up
Keller et al.[32]	Twice daily for 14 days		1 h after food intake, twice daily for 10 min	A mixture of L. reuteri DSM 17938 (1 × 108 CFU) and L. reuteri ATCC PTA 5289 (1 × 108 CFU)/chewing gum		VSC level, OLT score	Baseline and 2 weeks
Han et al.[33]	Once daily for 8 weeks		One tablet every night before bedtime, after brushing teeth.	1.0 × 108 colony forming units (CFU)/tablet of W. cibaria CMU		VSC levels and bad breath improvement (BBI)	Baseline, 4 weeks and 8 weeks
Benic et al.[34]	2 lozenges daily for 1 month		Suck two lozenges each day, after brushing teeth in the morning and in the night	3.6 × 109 S. salivarius colony forming units (CFUs)/lozenge		Plaque Index (PI), Gingival Index (GI), volatile sulfur compound (VSC) levels, and plaque samples	Baseline, 1-month intervention and 3 months intervention free
Lee et al.[35]	Once daily for 8 weeks		Melt and suck one tablet in the mouth every night before bedtime after brushing teeth.	1.0 · 108 colony-forming units (CFU)/g of W. cibaria CMU/ tablet		OLT scores, VSC level and BBI	Baseline, 4 weeks and 8 weeks
He et al. (2020)[36]	Twice daily for 30 days		Suck a tablet twice daily following tooth brushing in the morning and evening	1 × 109 colony-forming units (CFU) of S. salivarius K12/ lozenge		OLT scores and VSC levels, area of tongue coating (Ta), thickness of tongue coating (Tt), plaque index (PLI), PD, BI, and BOP%.	Baseline and on the 1st, 7th, and 14th day following the 30-day course of tablet-taking
Ding et al.[37]	Three times daily for 28 days		Pour powder in the mouth, melt with saliva, and leave for 1 min before swallowing	L. plantarum CCFM1214 and L. salivarius CCFM1215 2 × 109 CFU/ packet of powder		VSC SCORES GI, GBI, PLI, CI	Baseline and on the 7th, 14th, 28th, and 35th days

^a:Number of participants in the randomization process.

^b:Number of participants whose data were analyzed.

Only three studies[32,36,37] reported on the caries status of participants, including only caries-free individuals. The other studies did not provide information on dental caries status. Smoking status was considered in two studies,[36,37] which included only non-smokers. Han et al.[33] implemented a temporary smoking ban, while the remaining studies did not mention smoking behavior.

STUDY DESIGN AND EVALUATION PERIOD

All six studies followed a randomized, placebo controlled clinical trial design. Among these, Keller et al.[32] used a crossover design, while the others employed parallel-group formats. Evaluation durations varied from 14 days[32] to 4 weeks[34,36] and up to 8 weeks.[33,35] Follow-up evaluations were reported in three studies[34,36,37] to assess the persistence of effects.

INTERVENTION TREATMENT AND PROCEDURES

The administration methods for the probiotics varied as follows: (a) lozenges,[34,36] (b) tablets,[33,35] (c) chewing gum,[32] and (d) powder.[37] Regarding the specific probiotic strains, two studies used W. cibaria CMU,[33,35] two studies used S. salivarius,[34,36] one study used a mixture of L. reuteri DSM 17938 and L. reuteri ATCC PTA 5289,[32] and one study used a mixture of L. plantarum CCFM1214 and L. salivarius CCFM1215.[37]

In terms of timing, four studies explicitly stated that probiotics were to be consumed after regular oral hygiene practices and brushing.[33,34,35,36] However, Keller et al.[32] and Ding et al.[37] did not specify the timing of oral hygiene before probiotic intake. Furthermore, except for two studies,[32,37] all studies encouraged participants to maintain their normal hygiene routine and prohibited the use of supplementary hygiene methods, such as mouthwashes, or the intake of any other probiotics unrelated to the study.

DEFINITION OF HALITOSIS

All studies defined minimum thresholds for halitosis as an inclusion criterion. Keller et al.[32] used OLT ≥1 (barely perceptible odor); He et al.[36] required OLT ≥2 and VSC ≥150 ppb; Ding et al.[37] set a VSC threshold of ≥200 ppb. Han et al.[33] and Lee et al.[35] both applied a minimum baseline VSC of ≥1.5 ng/10 mL. The use of different units (ng/10 mL vs. ppb) stems from the measurement device: Han et al.[33] and Lee et al.[35] employed the Oral Chroma, while the remaining studies used a Halimeter. Benic et al.[34] did not state a specific diagnostic cutoff but implied a VSC ≥150 ppb.

All studies utilizing portable halitosis detectors followed device-specific protocols, typically averaging three readings to determine the final VSC value. For the Oral Chroma, participants remained silent for 3–5 min while holding a gastight syringe in the mouth. A 1-mL air sample was then extracted and injected into the device. For the Halimeter, Benic et al.[34] instructed participants to slightly open their mouths and breathe through their noses, while a straw was held in front of the mouth without touching any tissue. Keller et al.[32] required participants to rinse with 10 mL of L-cysteine for 1 minute; VSC levels were then measured after 10 min.

Three studies reported OLT scores. Keller et al.[32] and Lee et al.[35] used the Greenman scale (0–5), while He et al.[36] employed the Rosenberg scale (0–5). For the OLT procedure, Lee et al.[35] and He et al.[36] had participants close their mouths before exhaling toward the examiner’s face at a 10 cm distance. In contrast, Keller et al.[32] had participants exhale through a 30-cm glass tube, with the opening 5 cm from the clinician’s nose.

METHODOLOGICAL QUALITY OF INCLUDED STUDIES

Risk-of-bias assessments for each study are shown in Table 3. One study[34] was rated as low risk, one[37] as high risk, and four[32,33,35,36] as having some concerns. These ratings were based on RoB 2.0 criteria.[38,39]

Table 3.

Risk of bias of included randomized controlled clinical trials

Author (year)	Randomization	Deviations from intended interventions	Missing outcome data	Measurement outcome	Selection of reported result	Overall risk of bias (low, some concerns, high)
Keller et al.[32]	Low	Some concerns	Low	Low	Low	Some concerns
Han et al.[33]	Low	Some concerns	Some concerns	Low	Low	Some concerns
Benic et al.[34]	Low	Low	Low	Low	Low	Low
Lee et al.[35]	Low	Some concerns	Some concerns	Low	Low	Some concerns
He et al.[36]	Low	Some concerns	Low	Low	Low	Some concerns
Ding et al.[37]	Some concerns	High	High	Low	Low	High

METHODOLOGICAL QUALITY OF INCLUDED STUDIES

Among the included studies, one study was classified as having a low risk of bias,[34] one had a high risk of bias,[37] and the remaining four studies[32,33,35,36] were assessed as having some concerns. Table 3 presents the risk of bias for each individual domain, as assessed using the Cochrane Risk of Bias Tool 2.0[40,41], as detailed below:

RISK OF BIAS ARISING FROM THE RANDOMIZATION PROCESS

All studies were assessed as having low risk of bias in this domain, indicating that the allocation sequence was adequately concealed and randomized. Baseline imbalances were not reported before participants were enrolled and assigned to interventions, except for one study,[37] which did not provide information on the randomization process. However, no baseline imbalances were detected, suggesting no significant issues. Therefore, this study was classified as having “some concerns” regarding randomization.

RISK OF BIAS DUE TO DEVIATIONS FROM INTENDED INTERVENTIONS

All studies reported a double-blind intervention procedure with no major deviations. One study[34] performed an intention-to-treat (ITT) analysis, accounting for any missing outcome data such as patient dropouts during the intervention period, even though no dropouts occurred. This study was thus rated as having a low risk of bias. However, five studies implied a per-protocol analysis. Patient dropouts were minimal in four of these studies, resulting in low potential for substantial impact on the overall results, and were therefore rated as having “some concerns” in this domain. One study[37] had significant dropout rates in the control group, which were disproportionate to the probiotic group. This study was assessed as having a high risk of bias. Overall, the studies did not deviate from the original minimum number of participants required for achieving the desired study power in both the control and probiotic groups.

RISK OF BIAS DUE TO MISSING OUTCOME DATA

Four studies[32,33,34,36] had complete data for all or nearly all recruited patients. In one study,[35] two patients were excluded due to “abnormal data,” although no further explanation was provided. Since the proportions of missing outcome data were equal between the groups, this study was rated as having “some concerns.” In another study,[37] the proportions of missing outcome data were unequal between groups, leading to a high risk of bias for this study.

RISK OF BIAS IN THE MEASUREMENT OF OUTCOMES

In all studies, the method of measuring the outcomes was appropriate and consistent between groups. Additionally, the outcome assessors were either unaware of the intervention received by participants or the assessment of the outcome could not have been influenced by knowledge of the intervention received. This ensures minimal bias in outcome measurement.

RISK OF BIAS IN THE SELECTION OF REPORTED RESULTS

All studies were assessed as having low risk of bias in this domain. The data were analyzed according to a prespecified plan finalized before unblinded outcome data were available. Furthermore, the results were unlikely to have been selected based on the outcomes or from multiple eligible outcome measurements or analyses within the outcome domain.

This assessment of methodological quality underscores the overall robustness of most studies, while highlighting areas where some concerns or higher risks of bias may affect the interpretation of the results.

EFFECT OF PROBIOTICS ON HALITOSIS PARAMETERS

Participants across the selected studies were generally asked to abstain from eating, drinking, or performing oral hygiene practices prior to assessment. For example, He et al.[36] required a fasting interval of 2 hours before testing. However, studies by Lee et al.,[35] He et al.,[36] and Keller et al.[32] did not specify oral hygiene requirements for the assessment morning. In contrast, Ding et al.[37] imposed dietary restrictions, including avoidance of odor-inducing substances like garlic for 24 h pre-assessment. Benic et al.[34] did not mention fasting protocols. An overview of these procedural differences is available in Table 4.

Table 4.

Methods of assessing halitosis parameters across studies

Author (year)	Instructions prior to halitosis assessment	Examiners	Scoring System of OLt scores and/or vsc levels	Method for obtaining samples for assessment
Keller et al. (2012)[32]	No eating, drinking and oral hygiene measures on the morning of the tests and no eating spicy food on the evening before	Two aligned organoleptic judges	OLT: Glass tube method 0–5 scoring (Greenman scoring system) 0: No odor 1: questionable odor 2: slight malodor 3:moderate malodor 4: strong malodor 5: extremely strong malodor VSC: Halimeter	After initial breath measurements, the participants were instructed to rinse with 10 mL of L-cysteine suspension for 1 min to create a standardized malodor. The malodor was then assessed by the Halimeter 10 min after rinsing.
Han et al.[33]	No eating after brushing teeth in the evening until the morning of the measurement.	One aligned examiner	VSC: Oral Chroma	No talking 5 min before measurements and close the mouth for 30 s with a gastight syringe in the mouth.
Benic et al.[34]	Not reported	One aligned examiner	VSC: Halimeter	Open mouth slightly, and a straw connected to the Halimeter was placed inside the mouth without touching the teeth, tongue, or other tissues. The patients were instructed to breathe from the nose, not to blow or suck nor close lips.
Lee et al.[35]	No eating or drinking beverages or food and no oral hygiene measures on the morning of each visit and no food that could affect oral malodor on the evening before	One aligned trained examiner	OLT: 0-5 (Greeenman scoring system) 0: No odor 1: questionable odor 2: slight malodor 3:moderate malodor 4: strong malodor 5: extremely strong malodor VSC: Oral Chroma	OLT: Exhale briefly through the mouth, at a distance of 10 cm from the nose of the examiner. VSC: No talking 5 min before measurements and close the mouth for 30 s with a gastight syringe in the mouth.
He et al.[36]	No onions, leeks, garlic and alcohol 24 h before the assessment. No eating, drinking, chewing gums, or oral hygiene 2 h before the assessment	One aligned examiner	OLT: 0–5 (Rosenberg) 0: None 1: barely noticeable 2: slight but clearly noticeable 3: moderate 4: strong offensive 5: extremely foul VSC: Halimeter	OLT: Close mouth for 1 min, exhale slowly form mouth to clinicians face at 10 cm distance.
Ding et al.[37]	No irritant such as garlic or shallots 24 h before the assessment	Two aligned examiners	VSC: Halimeter	Not reported.

All trials used handheld sulfur detection devices such as the Halimeter® or Oral Chroma™ to assess VSC concentrations. OLT scoring was reported in only three of the studies. Keller et al.[32] observed a marked improvement in OLT scores without corresponding significant shifts in VSC levels. Lee et al.[35] noted significant changes in both OLT and VSC at the 4-week mark; however, these changes were not sustained at week 8. In contrast, Han et al.[33] reported a significant decrease in VSCs, particularly H₂S and CH3SH, with the most notable effects evident at the 8-week endpoint. Benic et al.[34] demonstrated continued suppression of VSC levels even 3 months after the intervention, suggesting prolonged benefits of probiotic use. He et al.[36] also reported reduced OLT and VSC values post 30-day intervention, with these changes maintained at least 14 days beyond the treatment period, albeit with signs of partial recurrence. Similarly, Ding et al.[37] recorded a significant drop in VSC values after 2 weeks, which persisted 1 week after the 28-day course ended (day 35). Due to considerable variability in protocols, probiotic strains, treatment durations, and assessment parameters, a meta-analysis was deemed inappropriate.

ASSESSMENT OF OTHER HALITOSIS PARAMETERS

Two studies[33,35] evaluated the BBI score[40] as an additional primary outcome. Both detected statistically significant improvements in the probiotic group versus placebo at the 8-week mark, though not at week 4. Furthermore, BBI scores tended to decline after completion of the intervention.

ASSESSMENT OF SECONDARY ENDPOINTS

Three studies[34,36,37] explored additional intraoral indicators. He et al.[36] examined TCS and periodontal markers like PI, PD, and BoP, finding no significant intergroup differences, despite some improvements in the probiotic arm. Similarly, Benic et al.[34] noted stable periodontal health in both groups during and after the study, with no significant variation in PI or GI. In contrast, Ding et al.[37] observed statistically significant reductions in CI and GI scores after 28 days, while PI and BoP remained unchanged.

MICROBIOLOGICAL OUTCOMES

Microbiological analysis was conducted in four studies. Han et al.[33] and Lee et al.[35] both found significant differences between the groups in the changes in the proportion of Weissella cibaria from baseline to 8 weeks, based on tongue samples. Ding et al.[37] conducted saliva sampling and found a relatively higher abundance of Proteobacteria and lower levels of Fusobacteria in the probiotic group compared to the control group, along with a significant reduction in Fusobacterium nucleatum. However, Benic et al.[34] did not observe any significant changes in the presence of S. salivarius between the treatment groups and time periods, based on tooth plaque samples. The remaining two studies did not perform bacterial quantitative analyses.[33,36]

SAFETY AND TOLERABILITY

Probiotics were found to be very safe when incorporated into daily regimens. No critical adverse effects were noted during the trials, and participants experienced no pain, discomfort, or adverse reactions during the intervention periods. Compliance rates were high, suggesting that probiotics were well-tolerated. Given the absence of adverse effects, these findings support the safety of probiotics as an adjunct to oral hygiene programs, with evidence indicating their utility and good tolerability in the management of bad breath.

DISCUSSION

This systematic review aimed to evaluate the effectiveness of probiotics in managing oral halitosis, focusing primarily on outcomes such as reductions in VSC levels and improvements in OLT scores. The findings suggest that probiotics, particularly strains like Streptococcus salivarius K12 and Weissella cibaria, show promise in reducing VSC levels and improving OLT scores when compared to placebo. These effects are likely driven by alterations in the oral microbiota, promoting a more favorable microbial composition that reduces the production of malodorous compounds. However, the heterogeneity in the study designs, probiotic strains used, and outcome measures limits the ability to draw definitive conclusions. While some studies reported statistically significant improvements, others observed minimal or no effects, highlighting the variability in study quality, intervention protocols, and assessment methods.

STRENGTHS AND IMPLICATIONS

This review is one of the few that comprehensively evaluates the role of probiotics in managing oral halitosis, specifically among patients without active periodontitis. By employing a rigorous search strategy across multiple databases, trial registries, and gray literature, the review minimizes publication bias and provides a broad overview of the available evidence. The findings support the potential of probiotics as a non-invasive, adjunctive therapy for managing halitosis. Given the increasing consumer preference for natural and non-pharmaceutical interventions, these results hold clinical relevance and public health significance. Probiotics could serve as an accessible and cost-effective option for individuals looking to manage halitosis without relying on chemical-based mouthwashes or antibiotics.

LIMITATIONS

Despite its strengths, this review has several limitations.

First, the absence of a meta-analysis limits statistical power. While meta-analyses provide more precise effect estimates, the substantial heterogeneity of included studies made this approach unfeasible. Future research should prioritize methodological consistency to enable meta-analytical evaluation. Second, significant heterogeneity across studies complicates direct comparisons. Variations in design, probiotic strains, dosage, outcome measures, and methodological parameters introduce inconsistencies, reducing the reliability. Standardized protocols are essential for improving cross-study comparability. Third, the limited number of studies included, combined with small sample sizes and short follow-up durations, hampers the ability to generalize findings and evaluate long-term effectiveness. Small sample sizes increase the risk of Type II errors, while short follow-up periods make it difficult to assess the persistence of benefits. Fourth, concerns about the methodological quality of the studies reduce their reliability. One or more studies showed a high risk of bias, particularly regarding deviations from planned interventions and missing data, which could distort the overall conclusions. More rigorous quality assessments are essential. Additionally, the review lacks an in-depth analysis of dose-dependent effects. Variations in dosage regimens may influence the efficacy, and future studies should explore optimal dosages for maximum benefit. Lastly, potential confounding factors—such as baseline oral health, smoking, diet, and compliance—were not fully addressed. Future research should control for these variables to better isolate probiotic effects.

COMPARISON WITH EXISTING LITERATURE

Compared to previous studies,[41,42,43,44,45] this review established more robust inclusion criteria. Notably, it included a wider age range, whereas some studies[43,44] focused exclusively on adults, limiting the generalizability of the findings. Additionally, this review prioritized randomized controlled trials (RCTs) to assess probiotics as monotherapy, ensuring clearer insights into their specific efficacy for intraoral halitosis. Using probiotics alongside adjunctive treatments, such as chlorhexidine,[43,45] could confound results by independently altering the oral microbiota.

Furthermore, this review emphasized periodontal health to provide a more balanced understanding of the efficacy of probiotics. Unlike some studies[43,44] that included patients with periodontitis—potentially skewing results due to the inherent impact of inflamed gums and periodontal pockets on halitosis—our approach enhances result accuracy.

Another strength of this review is its inclusion of RCTs that investigated various probiotic strains and administration methods. This allowed for a comparative assessment of the efficacy across strains and provided valuable insights into safety considerations related to probiotic delivery vehicles, particularly in identifying potential adverse effects. In contrast, a previous systematic review[41] focused exclusively on Lactobacillus strains, significantly limiting the scope of available probiotic alternatives.

Finally, this review explored secondary outcomes and microbiological findings—an aspect often overlooked in prior studies. Addressing these factors enhances our understanding of the broader implications of probiotic use for halitosis management.

FUTURE DIRECTIONS

Future studies should prioritize high-quality RCTs with larger sample sizes, consistent outcome measures, and extended follow-up periods (ideally 12 months or more). Long-term research is essential for assessing the enduring effects of probiotics and their potential in preventing halitosis recurrence. Comparative trials examining different strains, dosages, and delivery methods will be valuable in identifying the most effective probiotic formulations for clinical applications. Additionally, greater emphasis should be placed on thoroughly documenting adverse effects to better understand the safety profile of probiotics, especially with prolonged use. While probiotics are typically considered safe, comprehensive reporting of side effects across diverse demographic groups will improve their clinical utility and expand their role in halitosis management. Moreover, incorporating advanced techniques such as metagenomic sequencing to explore the effects of various probiotic strains on the oral microbiome will provide valuable insights. This approach could assist in identifying the most beneficial strains and optimizing the dosing and delivery methods for sustained effectiveness in managing halitosis.

CONCLUSION

This systematic review highlights the promising potential of probiotics as a biological alternative for managing oral halitosis, supported by significant reductions in VSC levels, improved OLT scores, and alterations in the oral microbiota, as observed in the included trials. The review not only suggests that probiotics can enhance oral breath but also emphasizes their safety and good patient compliance. However, the variability in study designs, probiotic strains, dosages, and evaluation methods limits the ability to draw definitive conclusions.

Probiotic monotherapy shows significant reductions in VSC levels and OLT scores, with a potential long-term role in halitosis management. Future trials should aim for strain-specific standardization and extended follow-ups to establish clinical utility and standardize treatment protocols. Despite these limitations, the findings underline the potential of probiotics as a complementary approach to oral health management.

CONFLICTS OF INTEREST

There are no conflicts of interest.

AUTHOR CONTRIBUTIONS

G.P. contributed to the study conception, data collection, data acquisition and analysis, data interpretation, and manuscript writing. C.N. contributed to the study conception, data collection, data acquisition and analysis, data interpretation, and manuscript writing. S.D. contributed to the study conception, data collection, data acquisition and analysis, data interpretation, and manuscript writing. K.P. contributed to the study conception, data collection, data acquisition and analysis, data interpretation, manuscript writing, and additional roles in ensuring the manuscript met publication standards. All authors reviewed and approved the final version of the manuscript for publication.

ETHICAL POLICY AND INSTITUTIONAL REVIEW BOARD STATEMENT

Compliance with ethical standards.

PATIENT DECLARATION OF CONSENT

Not applicable.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

ABBREVIATIONS

• BBI Bad Breath Improvement

• BI Bleeding Index

• BoP Bleeding on Probing

• CH₃SH Methyl mercaptan

• CHX Chlorhexidine

• CI Calculus Index

• CPC Cetylpyridinium chloride

• GI Gingival Index

• H₂S Hydrogen sulfide

• ITT Intention-to-Treat

• OLT Organoleptic

• OSF Open Science Framework

• PD Probing Depth

• PI Plaque Index

• PICOS Population, Intervention, Comparison, Outcome, and Studies

• PRISMA Preferred Reporting Items for Systematic Reviews and Meta-Analyses

• RCTs Randomized Controlled Clinical Trials

• RoB Risk of Bias

• S. salivariusStreptococcus salivarius

• TCS Tongue Coating Scores

• VSCs Volatile Sulfur Compounds

• W. cibaria Weissella cibaria

• Zn Zinc

Funding Statement

Nil.