Effects of Botulinum Toxin Injection on Masticatory Muscle Activity in Sleep Bruxism: A Systematic Review and Meta-Analysis

Background/Objectives: Bruxism, encompassing both awake (AB) and sleep bruxism (SB), is a prevalent condition associated with involuntary jaw muscle activity, leading to tooth wear and muscle hypertrophy. Botulinum toxin type A (BoNT-A) has emerged as a potential treatment by reducing masticatory muscle hyperactivity. This systematic review aimed to evaluate the efficacy of administering BoNT-A injections into the masticatory muscles for reducing bruxism-related muscle activity, as measured by any validated instrumental or questionnaire-based method. Methods: A systematic search of PubMed, Scopus, and Web of Science was performed according to PRISMA guidelines, with the search updated through July 2026. Eight studies evaluating bruxism/masticatory muscle activity after BoNT-A injection, using electromyography (EMG), polysomnography, or validated questionnaires/clinical global impression scales, met the inclusion criteria. Five placebo-controlled randomized controlled trials were synthesized quantitatively; a meta-analysis using a random-effects (DerSimonian–Laird) model, chosen a priori given the anticipated methodological heterogeneity across studies and confirmed by a fixed-effect model computed as a sensitivity analysis, was used to calculate the pooled Hedges’ g standardized mean differences with 95% confidence intervals (CIs) for bruxism/muscle-activity outcomes. Three additional studies with non-placebo designs—two active-comparator RCTs and one large observational study—were synthesized narratively because their designs (no placebo arm) were not compatible with pooling. Risk of bias was assessed with the Cochrane Risk of Bias 2.0 tool and the Newcastle–Ottawa Scale. Results: Because the five placebo-controlled trials captured two related but distinct constructs of muscle activity, construct-specific estimates are presented as the primary quantitative findings: a fixed-effect pooled Hedges’ g of −0.70 (95% CI: −1.04 to −0.37) for studies measuring bruxism event frequency, and −1.31 (95% CI: −1.95 to −0.68) for studies measuring EMG signal amplitude, in patients with sleep bruxism (SB) compared with placebo. As a secondary, exploratory summary across both constructs, the overall random-effects pooled estimate was Hedges’ g = −1.09 (95% CI: −1.65 to −0.54), with moderate-to-substantial heterogeneity (I2 = 58.6%). The formal test for the numerical difference between these two construct-specific estimates was not statistically significant (p = 0.096), reflecting the small number of studies per subgroup; the overall pooled estimate is therefore best interpreted as a secondary, composite signal across related constructs. The three non-pooled studies—two active-comparator RCTs and a large real-world observational series (number of partecipants = 304)—independently reported reductions in masticatory muscle EMG amplitude after BoNT-A injection, consistent in direction with the pooled estimate. In contrast, of the six studies that also assessed bruxism frequency by questionnaire, clinical global impression, or PSG scoring, only one found a clear BoNT-A-specific benefit; the remaining five found no significant difference from the comparator, even when an EMG-based measure from the same participants did show a BoNT-A-specific reduction. Mild and transient adverse effects were reported, with no serious events. The certainty of evidence was rated as low-to-moderate regarding the precise magnitude of EMG-based muscle activity reduction (downgraded for construct heterogeneity and imprecision) and very low regarding any corresponding reduction in questionnaire- or clinically-assessed bruxism frequency, and low concerning the short-term safety of BoNT-A administration (downgraded from the RCT-default starting point for imprecision, given the small number of controlled participants and short follow-up, and for indirectness, given heterogeneous dosing protocols across studies). Conclusions: BoNT-A injections into the masticatory muscles consistently reduce EMG-detectable masticatory muscle activity in patients with SB, although the magnitude of this effect varies depending on whether event frequency or signal amplitude is used to quantify it. This EMG-measured reduction did not consistently correspond to a reduction in bruxism frequency as assessed by questionnaires, clinical global impression, or PSG scoring, and the two types of measures should not be assumed interchangeable when interpreting treatment response. BoNT-A consistently reduces EMG-detectable masticatory muscle activity and, on this mechanistic basis, may be considered a candidate adjunctive therapy, particularly for individuals unresponsive to conservative treatments; claims of clinical efficacy should specify which type of bruxism measure they refer to, since this review does not provide direct evidence of benefit on pain, function, or other bruxism-related symptoms. Future research should focus on standardizing injection protocols and outcome measures—ideally reporting instrumental and questionnaire-based outcomes concurrently—and on evaluating long-term safety and efficacy.