A detailed explanation of the face-slimming effect and safety of masseter Botox
Masseter Botox is widely recognized as a cosmetic treatment for refining the facial contour and obtaining a so-called "face-slimming effect." However, it is important to grasp accurate, science-based information about its mechanism, duration, and safety. In this article, based on data from the latest randomized controlled trials (RCTs) and large-scale meta-analyses, we explain in detail the effects and risks of masseter Botox.
1. What is masseter Botox
Masseter Botox is a treatment that injects botulinum toxin (Botulinum toxin type A: BoNT-A) into the muscle called the "masseter" on the side of the lower jaw (Nobre et al. 2024) [1, 2]. Botulinum toxin is a neurotoxin produced by the bacterium Clostridium botulinum, and by blocking the release of the neurotransmitter acetylcholine at the neuromuscular junction, it causes temporary muscle paralysis and the accompanying muscle atrophy (Nobre et al. 2024; Ozdemir Cetinkaya et al. 2024) [2, 3].
The perception of facial beauty differs by culture, but generally, in women, compared with men, an "inverted triangle" face with a narrow lower face tends to be considered attractive (Nobre et al. 2024) [4]. However, depending on the shape of the mandible, the amount of subcutaneous tissue, and hypertrophy of the masseter, the lower face widens and a "square face" can form (Nobre et al. 2024) [4]. The masseter is a quadrilateral muscle attaching from the zygomatic arch to the mandibular ramus, and together with the temporalis and the medial and lateral pterygoid muscles, it is an important muscle responsible for mastication (Ozdemir Cetinkaya et al. 2024) [5].
The state in which this masseter enlarges painlessly and asymptomatically is called "masseter hypertrophy." Masseter hypertrophy occurs unilaterally or bilaterally and is often seen in people in their 20s to 40s (Nobre et al. 2024) [6]. The cause of masseter hypertrophy is not completely elucidated, but it is thought to be related to psychological stress, chronic bruxism (teeth grinding or clenching), minor trauma, or chewing habits (Nobre et al. 2024; Ozdemir Cetinkaya et al. 2024) [6, 7].
In a survey of 80 patients who visited an actual dermatology clinic (74 women, 6 men, mean age 31.20±6.71 years), 52 patients (65%) received treatment for the purpose of lower-face contouring (face-slimming effect) (Ozdemir Cetinkaya et al. 2024) [8, 9]. In addition, many of these patients also had concurrent bruxism, and of the total, 62 patients (77.5%) were found to have a history of bruxism (Ozdemir Cetinkaya et al. 2024) [9]. Thus, masseter Botox is often a treatment performed for both cosmetic contouring and the functional improvement of relieving teeth grinding and clenching (Ozdemir Cetinkaya et al. 2024) [10-12].
2. The masseter-reduction effect shown by RCTs
That masseter Botox reduces muscle volume and brings a visual face-slimming effect has been scientifically demonstrated by rigorous randomized controlled trials (RCTs). In a triple-blind RCT conducted on 26 female patients suffering from masseter hypertrophy, 75 U (units) of Abobotulinum toxin (a type of botulinum toxin preparation) was injected into each patient's left and right masseter, respectively (Nobre et al. 2024) [1].
📊 Masseter Botox effect data (Nobre et al. 2024 / Ozdemir Cetinkaya et al. 2024)
65%
of patients visited for face-slimming purposes(52 of 80)
75U
RCT standard dose(AbobotulinumtoxinA)
77.5%
had bruxism (teeth grinding)(62 of 80)
Significant difference
Muscle thickness reduction after a single injection(confirmed by ultrasound)
Source: Nobre et al. 2024 (triple-blind RCT, 26 people) / Ozdemir Cetinkaya et al. 2024 (80-person retrospective study)
In this study, muscle thickness was measured in detail in millimeters (mm) using ultrasound (US) (Nobre et al. 2024) [1, 13]. As a result, it was confirmed that after a single Botox injection, the muscle thickness significantly decreased from baseline (before treatment) (Nobre et al. 2024) [1]. In addition, when the muscle's electrical activity (microvolts: μV) was evaluated using surface electromyography (EMG), the muscle activity of the right masseter dropped dramatically from a baseline of 273.72±174.64 μV to 48.63±20.29 μV one month later, and the left masseter also showed a significant decrease from a baseline of 243.23±145.11 μV to 49.44±51.85 μV one month later (Nobre et al. 2024) [14, 15]. This dramatic decrease in muscle activity is the direct factor causing muscle atrophy (size reduction).
The muscle-reduction effect of Botox is also clearly shown in other research data conducted in the past. One retrospective survey reported that when 100 U to 140 U of AbobotulinumtoxinA was injected into the masseter, the average size of the masseter shrank substantially from 13.32 mm to 9.94 mm (Ozdemir Cetinkaya et al. 2024) [16, 17]. In addition, another study with 4 years of follow-up reported that by injecting 30 U of OnabotulinumtoxinA per side, the masseter size shrank by an average of 12% at week 12 (about 3 months later), and that by performing periodic repeated dosing, the contour-improvement effect was further emphasized (Ozdemir Cetinkaya et al. 2024) [17, 18]. These figures confirm that masseter Botox reliably reduces the physical volume of the masseter.
3. The duration of the face-slimming effect
The face-slimming effect from masseter Botox is not permanent; as time passes, nerve reinnervation occurs, and the muscle's function and thickness recover. In the aforementioned RCT (Nobre et al. 2024), when the course of the group that received only a single Botox injection was tracked, it was observed that the muscle thickness began to recover 3 months after the injection (Nobre et al. 2024) [19]. In the muscle-activity evaluation by electromyography as well, 3 months after a single injection the activity began to rise to 88.39±48.89 μV in the right masseter and 78.23±58.86 μV in the left masseter, and 6 months later it recovered to 178.83±93.15 μV in the right masseter and 163.12±118.70 μV in the left masseter (Nobre et al. 2024) [14, 15]. In addition, a past systematic review showed that biting force and occlusal force return to normal in about 3 months (Nobre et al. 2024) [20].
However, the most important finding of this RCT concerns the durability of the effect when "multiple injections (booster injections)" are performed. In the group (G2) that received a second Botox injection (booster) 3 months after the first injection, the muscle thickness did not recover and remained in a reduced state even at 6 months after the first injection (3 months after the second injection) (Nobre et al. 2024) [1, 19]. Regarding muscle activity as well, in the group that received the second injection, at 6 months the right masseter remained at 81.77±48.69 μV and the left masseter at 74.49±48.29 μV, maintaining a significantly lower state compared with the placebo-injected group (Nobre et al. 2024) [14, 15]. This shows that repeated Botox dosing delays the functional recovery of the neuromuscular junction, and the muscle-atrophy effect lasts for a longer period (Nobre et al. 2024) [20].
Therefore, it can be concluded that to maintain the face-slimming effect stably over a long period, it is extremely effective to repeat multiple injections at appropriate intervals of about 3 months to half a year, rather than a one-time treatment.
4. Side effects and safety
Masseter Botox is a minimally invasive treatment that does not use a surgical scalpel and is considered relatively safe, but it is not completely harmless, and a certain risk of side effects exists. In a large-scale meta-analysis (including 32 RCTs) evaluating the safety of Botox injections for facial rejuvenation (mainly including wrinkle treatment) in 9,669 patients, compared with the placebo group, the overall relative risk (RR) of an adverse event (side effect) occurring was 1.53 (95% CI, 1.33-1.77, p < 0.001), and a significant increase in risk from Botox injection was found (Gostimir et al. 2023) [21-23]. Focusing on individual side effects, the relative risk of headache was 1.45 (95% CI, 1.13-1.84, p = 0.003), the relative risk of facial paralysis was 2.42 (95% CI, 0.43-13.56, p = 0.316), and the relative risk of abnormal positioning of the eyebrows or eyelids (blepharoptosisand the like) was reported as 3.55 (p < 0.001) (Gostimir et al. 2023) [22, 24].
Regarding side effects specific to injection into the masseter (masseter Botox), in a retrospective study of 80 people, it was recorded that a total of 16 treatment-related side effects or complications occurred in 15 patients (18.8%) (Ozdemir Cetinkaya et al. 2024) [8, 25]. The specific incidence and breakdown are as follows.
- Bruising/internal bleeding at the injection site: 8 people (10%) (Ozdemir Cetinkaya et al. 2024) [26]
- Concurrent pain and headache at the injection site: 3 people (3.8%) (Ozdemir Cetinkaya et al. 2024) [27]
- Smile asymmetry: 2 people (2.5%) (Ozdemir Cetinkaya et al. 2024) [8, 28]
- Paradoxical bulging: 2 people (2.5%) (Ozdemir Cetinkaya et al. 2024) [8, 28]
"Paradoxical bulging" is a phenomenon in which, as compensation for the decline in deep-layer masseter function after injection, the superficial muscle fibers unaffected by the Botox contract excessively, and when the teeth are clenched, part of the muscle bulges out "prominently" like a frog (Ozdemir Cetinkaya et al. 2024) [29]. In addition, side effects such as smile asymmetry and left-right differences in how the corners of the mouth rise occur when the Botox diffuses to the surrounding muscles of facial expression, and since they greatly lower the patient's psychological satisfaction, caution is needed (Ozdemir Cetinkaya et al. 2024) [29].
Furthermore, the possibility of functional side effects must also be considered. Animal experiments and some clinical studies suggest that by repeating multiple Botox injections, the muscle type may change from a fatigue-resistant type (type IIa) to a fatigue-prone type (type IIb), or the muscle's contractile tissue may be replaced by adipose tissue (fatty infiltration) (Nobre et al. 2024) [2, 6]. It has also been reported that excessive reduction of occlusal force may adversely affect the bone quality of the mandible (bone density, etc.) (Nobre et al. 2024) [2]. To maintain masticatory function and muscle health, treatment at excessive frequency or excessive dose should be avoided.
5. The procedure flow and downtime
In actual procedures, there is a strict protocol to ensure safety while minimizing side effects. In the general injection technique adopted in the Turkish study, the "safety zone" into which the Botox is injected is set based on anatomical landmarks (Ozdemir Cetinkaya et al. 2024) [30]. Specifically, a rectangular region is set with a line 1 cm above the lower border of the mandible as the lower limit and an imaginary line connecting the corner of the mouth to the earlobe as the upper limit (Ozdemir Cetinkaya et al. 2024) [30].
Before the procedure, the patient is asked to clench the teeth firmly, and the anterior and posterior borders of the masseter are confirmed by palpation. Then, into the region 1 cm inside the boundary of the identified safety zone, Botox is injected evenly at multiple points (for example, 2 points in the lower part and 1 point in the upper-middle part, for a total of 3 points) (Ozdemir Cetinkaya et al. 2024) [30, 31]. In this study, when there was clear asymmetry (a difference in size) between the left and right muscles, an adjustment was made to inject an additional 0.1 ml (15.63 U) of Botox into the more hypertrophied side of the muscle to balance the two sides (Ozdemir Cetinkaya et al. 2024) [31, 32].
The downtime and side-effect recovery process are as follows.
- Bruising or internal bleeding:It often appears 1 to 2 days after the procedure, and even without special treatment it improves naturally in about 10 to 15 days (Ozdemir Cetinkaya et al. 2024) [26].
- Pain or headache:It appears 1 to 2 days after injection and usually improves in about 7 to 10 days. If the pain is severe, it is managed by taking a non-steroidal anti-inflammatory drug (NSAID) for a few days (Ozdemir Cetinkaya et al. 2024) [27].
- Paradoxical bulging or smile asymmetry:It is common for the patient to notice it 3 to 4 days after the procedure (Ozdemir Cetinkaya et al. 2024) [26, 27]. If bulging occurs, the symptom can be improved by performing a "touch-up"—injecting a small amount of Botox (7 U to 15 U) into the muscle fibers showing surface overactivity 10 to 15 days later (Ozdemir Cetinkaya et al. 2024) [27].
6. Who it is suited for / who it is not suited for
Masseter Botox treatment has clearly divided indications depending on the purpose and health condition. Based on the inclusion and exclusion criteria of clinical trials, we organize who it is suited for and who should avoid treatment.
Who it is suited for
- People bothered by a protruding jaw angle:People whose face looks square or whose lower face looks widened not due to the mandibular bone structure itself, but due to masseter development (muscle thickness). Through the muscle-atrophy action of Botox, a sharp contour (a slimmer face) can be obtained (Ozdemir Cetinkaya et al. 2024) [5, 9, 10].
- People suffering from bruxism (teeth grinding/clenching):People with unconscious teeth grinding or clenching. In fact, 77.5% of patients wishing for treatment had symptoms of teeth grinding. By suppressing the excessive activity of the muscle, a functional effect of reducing jaw fatigue and pain can also be expected (Ozdemir Cetinkaya et al. 2024) [8-10].
Who it is not suited for (people who meet exclusion criteria)
People who meet the following conditions cannot receive treatment, or require very careful judgment, because the risk of side effects is high or the effect cannot be correctly determined.
- Women who are pregnant or breastfeeding, or women planning pregnancy during the clinical trial period (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [33-36].
- People suffering from neuromuscular junction diseases such as myasthenia gravis, Lambert-Eaton syndrome, or amyotrophic lateral sclerosis (ALS), or systemic muscle/nervous-system diseases such as motor neuropathy (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [34-36].
- People taking medications that may interfere with neuromuscular transmission, such as aminoglycoside antibiotics, muscle relaxants, anticholinergics, or benzodiazepines (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [34-36].
- People with a past history of an allergic reaction or hypersensitivity to a botulinum toxin preparation (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [35, 36].
- People with marked facial asymmetry, people with a past history of facial nerve palsy, or people in whom blepharoptosis is seen (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [35, 36].
- People with an active skin disease, infection, or serious scar at the planned injection site (the face or around the jaw angle) (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [35, 36].
- People who have recently (within the past 6 to 12 months) undergone facial cosmetic surgery (a facelift, a thread lift, etc.), filler injection with hyaluronic acid or non-absorbable substances, or chemical peeling or laser treatment (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [33, 35, 36].
- People taking anticoagulants or antiplatelet drugs (except small low-dose aspirin), or people with a past history of a bleeding disorder (Ozdemir Cetinkaya et al. 2024; Gostimir et al. 2023; Nobre et al. 2024) [35, 37].
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7. Summary
Masseter Botox is widely performed as a minimally invasive cosmetic treatment that improves lower-face widening caused by excessive masseter development and brings an excellent face-slimming effect. From rigorous randomized controlled trial (RCT) and retrospective survey data, it has been demonstrated that botulinum toxin injection significantly reduces muscle thickness and electrical activity and is extremely effective not only for contouring but also for reducing bruxism (teeth grinding and clenching).
With a single injection, muscle thickness and function begin to recover in about 3 months, but by performing booster injections (multiple injections) periodically, the muscle-atrophy state is sustained, making it possible to maintain a longer-term face-slimming effect stably. On the other hand, it also carries the risk of side effects such as bruising, internal bleeding, headache, and even smile asymmetry and paradoxical bulging occurring at a certain rate (about 18.8%).
Furthermore, since the possibility of long-term functional side effects such as muscle being replaced by fat through repeated dosing has also been suggested, treatment at an excessive dose or frequency should be avoided. In treatment, accurate injection into the "safety zone" by a specialist well versed in anatomical structures, and appropriate aftercare when side effects occur (additional touch-up injections, etc.), are indispensable. It is strongly recommended to receive the procedure after fully consulting with a physician about whether there is an indication for treatment, taking into account one's own health condition, currently taken oral medications, past cosmetic procedure history, and so on.
References
- de Souza Nobre B, Rezende L, Barbosa Câmara-Souza M, et al. Exploring botulinum toxin’s impact on masseter hypertrophy: a randomized, triple-blinded clinical trial. Scientific Reports. 2024 DOI
- Ozdemir Cetinkaya P, Karaosmanoglu N, Özkesici Kurt B, et al. Functional and esthetic effects of botulinum toxin injection into the masseter muscles: evaluation of 80 patients from a dermatological perspective. International Journal of Dermatology. 2025 DOI
- Gostimir M, Liou V, Yoon M. Safety of Botulinum Toxin A Injections for Facial Rejuvenation: A Meta-Analysis of 9,669 Patients. Ophthalmic Plastic & Reconstructive Surgery. 2023 DOI
- Camargo C, Xia J, Costa C, et al. Botulinum toxin type A for facial wrinkles. Cochrane Database of Systematic Reviews. 2021 DOI
The author of this article
Hiromitsu Nakamura Physician
Zetith Beauty Clinic Ginza / Osaka / Fukuoka
He has a track record of research presentations at domestic and international conferences in Japan and is also involved in technical guidance and education across the entire Zetith Beauty Clinic. Specializing in precise cosmetic medicine based on anatomical grounds, he pursues natural results tailored to each individual's bone structure and tissue.