Botulinum Toxin for Jaw Clenching and the Masseter Muscle

What Is Botulinum Toxin for Jaw Clenching and the Masseter Muscle?

Masseter botulinum toxin is an injection of botulinum toxin type A into the chewing muscle (masseter) over the angle of the lower jaw. Together with the temporalis muscle at the temple, the masseter is one of the main muscles used for chewing; it sits at the side of the face, over the angle of the jaw. The everyday word used for this treatment comes from a brand name; here, it refers to the active ingredient, botulinum toxin. You can read how the toxin acts at the point where nerve and muscle meet on the Botulinum Toxin page.

Scientific studies look at this treatment for two separate purposes. The first is enlargement of the masseter muscle. Its cause is not fully known, and it looks like a soft swelling near the angle of the jaw; here the aim is to change the contour of the lower face by making the muscle smaller. The second is teeth grinding and clenching (bruxism). In this case the aim is not appearance but reducing overactivity of the chewing muscles. Although the injection goes into the same muscle, the two purposes differ in their level of evidence, how they are assessed and what can be expected.

For enlargement of the masseter on both sides, an evidence-based review classed the evidence for improved facial contour and reduced muscle volume and thickness as level II. However, it is noted that the treatment is widely given off-label (outside the uses covered by the product’s licence or approval), and that high-quality evidence on the long-term safety of repeated injections is limited. According to another review, published in 2023, this use for a prominent masseter had not been officially approved in any country at that time. If you are considering the treatment, you can ask beforehand what the product to be used is licensed for in Türkiye.

The dose, injection points and dilution are tailored to each person. The doctor who gives the treatment decides these and discusses them with you at the consultation.

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Enlargement of the Masseter Muscle

If the width of the lower face comes from an enlarged masseter muscle, botulinum toxin aims to narrow the side contour of the lower face by temporarily weakening the muscle and reducing its volume. Randomised placebo-controlled trials (trials in which participants are randomly assigned to groups and one group receives an inactive substance) report short-term improvement; the evidence on long-term use and repeated treatments is more limited. This enlargement may not only be about appearance: some people can also have functional complaints, such as pain and tiredness in the chewing area and discomfort when chewing.

The evidence in this area has changed over time. A 2013 Cochrane review found no randomised or controlled trial of this use that met its criteria, and stressed the need for well-designed studies. A recent meta-analysis combining five randomised placebo-controlled trials with 536 participants reports that treatment was associated with reduced muscle thickness, improvement on a prominence scale and higher patient satisfaction. Even so, the authors note that the studies varied widely, that the evidence is not enough to define the best treatment protocol, and that studies with longer follow-up are needed.

In a multicentre, randomised, double-blind phase 3 trial (neither the participants nor the researchers knew which injection was given) with 376 participants, an improvement of at least two grades on the prominence scale at day 90 was seen in 51.2% of the treatment group and 2.2% of the placebo group; the authors reported that the effect lasted up to six months. In a phase 2b trial looking at a single injection, the rate of reaching the target grade was 90.6% and 91.3% in the two dose groups and 21.7% in the placebo group. These trials were done with a specific product and in people with marked muscle enlargement, so they cannot be used to predict what any one person can expect.

According to one review, this treatment is common in Asia, where a slimmer lower face is considered attractive. This is a beauty preference, not a standard; a prominent chewing muscle is not a disease in itself. The wish for treatment comes from your own judgement; whether the width of your lower face comes from the muscle or from another structure is determined at the consultation.

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Teeth Grinding and Clenching

In teeth grinding and clenching, botulinum toxin can reduce the measured activity of the chewing muscles. However, the evidence on its effect on how often bruxism occurs, on pain and on the teeth is mixed. According to an international expert consensus, bruxism is a condition dealt with in dentistry and the relevant fields of medicine; it is not only an aesthetic matter.

Bruxism is repetitive jaw-muscle activity that shows as clenching or grinding of the teeth and/or bracing or thrusting of the lower jaw. Bruxism during sleep and bruxism while awake are treated as two separate conditions. A more recent consensus statement proposes a graded system for assessment, based on the person’s own account, a clinical examination and measurements taken with a device. In other words, saying ‘I clench my teeth’ is a starting point, not a diagnosis.

According to an evidence-based consensus statement, there is moderate evidence that sleep bruxism is linked to psychosocial factors such as stress and mood, and to caffeine, alcohol and smoking. There is no established definitive treatment for bruxism, and there is no agreement on which symptoms should be treated. There is some evidence for oral splints and biofeedback (methods that make you aware of muscle activity through real-time signals); the evidence on botulinum toxin is conflicting.

In sleep bruxism, a recent meta-analysis combining five placebo-controlled trials reports that the injection consistently reduced the muscle’s electrical activity, measured by electromyography. However, of six studies that measured how often bruxism occurred using questionnaires, clinical assessment or sleep recordings, only one found a clear benefit, and the certainty of this result was rated very low. The authors note that the analysis does not provide evidence of a direct benefit for pain or function, and they discuss the treatment as an additional option, particularly for people who have not responded to non-invasive (conservative) methods.

The quality of the evidence in this area is also being questioned. An umbrella review (a review of systematic reviews) of 14 systematic reviews found the methodological reliability of all of them to be low or critically low. It stated that, although there were signs of reductions in pain, bruxism episodes and bite force, no firm conclusion could be drawn. Another study, which examined randomised trials, reports that a tendency to present results more favourably than they are (spin) was seen at a frequency of 59.4% in eight trials, and that results on masseter injections are debatable.

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What Changes in the Muscle and in Chewing?

Botulinum toxin reduces how strongly the masseter muscle contracts; a muscle that works less becomes thinner over time. This is why the change in the lower face appears over weeks, not immediately. As the effect wears off, the muscle is expected to work again; however, how far it returns to its previous state is still an open question.

In a study that followed 24 healthy adults, three-dimensional facial measurement showed a significant reduction in lower-face volume at weeks 4, 8 and 12; there was no impairment in chewing function as reported by the participants themselves. However, this is a small, quasi-experimental study, and the authors regard their findings as preliminary evidence. In the phase 3 trial that included people with marked masseter enlargement, the effect was reported to last up to six months.

Measured muscle function tells a different story. A systematic review of 44 human and animal studies on the effects on the chewing muscles reports that, even after a single injection, there were structural changes in the muscle, thinning and a change in muscle fibre type; bite force and muscle activity decreased, and recovery was uncertain. Because recovery is uncertain, the authors, who looked at the subject from the standpoint of orthodontics and jaw surgery, recommend avoiding repeated injections and keeping treatment in line with established consensus.

Another systematic review looked at the chewing muscles after masseter injection; only two of its 14 studies were done in humans. This review reports dose-dependent neuromuscular effects and incomplete recovery over time, but it stresses that strong clinical generalisations cannot be made because human data are very limited. In a small study that followed ten patients with imaging, the other chewing muscle at the temple (temporalis) became thicker as the masseter became thinner; the authors interpret this as a compensatory change.

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What Has Been Reported About the Jawbone

Whether botulinum toxin injected into the chewing muscle changes the jawbone is a question that does not yet have a clear answer. The findings conflict with each other, and the clinical importance of the reported changes is unclear. This is not a certainty; it is an uncertainty you should know about.

A meta-analysis that assessed human and animal studies together found that, in humans, the thickness of the outer layer of bone (cortex) decreased by about 6% in some areas of the jawbone after injection; there was no marked change in bone volume or density. In animal studies, however, marked bone loss was seen in the jaw joint area. The authors note that most studies did not look at repeated injections or high doses, and that more rigorous studies are needed on long-term effects.

A systematic review of human studies that used CT scanning found that five of seven studies reported reductions in bone volume, cortical thickness and density in areas such as the head of the jaw joint, the coronoid process and the upright part of the lower jaw; however, the samples were small and the quality of the studies very low. According to another systematic review that assessed five studies, most of the studies found no significant change; the notable finding was a difference in jaw-angle volume between people treated twice and those treated once, and no general pattern was found.

In a pre-specified subgroup of 123 people from a randomised, placebo-controlled phase 2 trial, no clinically meaningful change in jawbone density was found in healthy adults over 12 months after one or two treatments. On the other hand, a study assessed 39 young women and 38 post-menopausal women after 12 months; they had been managed with an oral splint and/or two botulinum toxin treatments in the chewing muscles. It reported that thinning of the muscle can affect the quality of the bone cortex, and that these effects were more marked in post-menopausal women. So if you have a condition that affects your bone health, it is important to mention it at your consultation.

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Anatomy and Safety

The masseter muscle lies at the side of the face, in the same area as branches of the facial nerve, the parotid gland (a salivary gland) and its duct, and the facial artery and vein. This is why where the injection is placed determines not only the result but also safety. The position of these structures and the internal layout of the muscle can vary from person to person.

An anatomical study of 24 cadavers described a triangular area in the masseter region where the facial nerve, parotid gland, parotid duct, facial artery and facial vein are least likely to pass. The same study divided the internal structure of the muscle into three types and reported that this structure varies between individuals; based on these findings, the authors propose an injection approach. However, this is a cadaver study, and in this study the proposed approach was not compared with clinical outcomes.

According to a comprehensive review of masseter enlargement, there are published data on injection technique, but there is not yet a firm consensus on the average dose. The injection site, depth and amount are decisions your doctor makes based on your anatomy, and they are discussed at the consultation.

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Limits of This Treatment

Masseter botulinum toxin is a muscle treatment: it temporarily weakens the muscle and does not change any other structure. If the width of your lower face comes not from the muscle but from bone structure, fatty tissue or skin laxity, the treatment does not affect that cause and will not meet your expectations. The effect is not permanent, and high-quality evidence on the long-term safety of repeated injections is limited. For someone who wants a more defined jawline, the aim is often to add contour rather than to make the muscle smaller. This is a different question and is covered on the Chin and Jawline Filler page.

For jaw clenching, the limit is clearer. There is no established definitive treatment for bruxism; botulinum toxin can reduce measured muscle activity, but it does not remove the cause of bruxism. A meta-analysis comparing a night guard (a mouth guard or splint worn at night) with botulinum toxin in sleep bruxism found no significant difference between the two in reducing pain. It stated that when preventing mechanical damage and maintaining chewing function are the priority, the night guard is the preferred conservative method.

Pain in the jaw joint and chewing muscles (temporomandibular disorder, TMD) is a separate issue and is not the same as clenching. A systematic review and meta-analysis on this, covering 14 randomised trials and 395 patients, concluded that botulinum toxin was not associated with better results than placebo for pain reduction, mouth opening or bite force. If you have jaw pain, limited mouth opening or wear on your teeth, these need to be assessed before an aesthetic treatment.

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Known Risks

Most side effects reported with masseter botulinum toxin are described as temporary. The main ones are: part of the muscle bulging when the teeth are clenched (paradoxical bulging), an asymmetric smile, a sunken look in the cheek and reduced chewing strength. The figures given here come from other studies and cannot be used to work out your personal risk.

Paradoxical bulging: A retrospective study looked at 46,250 people who had masseter treatment. Of the 223 people in whom an adverse effect was found at a follow-up visit and who were then monitored, 88.3% had paradoxical bulging. In this study, all adverse effects were temporary and resolved after an average of 5.3 months; there was no significant difference in time to resolution between those who received an intervention and those who were only monitored. Because the study covered only people who came back for follow-up and were monitored, it does not show how common these effects are overall.

Change in smile: In a retrospective study of 80 patients treated for lower-face contour or jaw clenching, 18.8% had a treatment-related adverse effect; smile asymmetry and paradoxical bulging were each reported in 2.5%. In a systematic review of randomised trials in sleep bruxism, one trial reported a cosmetic change in the smile in 15.4% of patients, and another reported pain at the injection site in 20% of participants; adverse effects were generally described as mild and temporary.

Sunken cheeks and chewing: According to a clinical study of injections for masseter enlargement, a sunken look in the cheek is one of the common unwanted aesthetic results of this treatment. Reduced bite force, on the other hand, is part of the expected effect of the treatment; a systematic review reports that recovery after this reduction is uncertain. You can find the general risks of botulinum toxin on the Botulinum Toxin page. Your doctor’s own protocol sets out which symptoms after treatment mean you should contact the clinic; these are explained to you at the consultation.

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