What is the masseter muscle, and what does botulinum toxin do in it?
The masseter is the muscle that runs from the cheekbone to the angle of the lower jaw and that you can feel at the back of the cheek when you clench your teeth. It is one of the strongest chewing muscles. When it works often and hard, it can thicken, just like other muscles that are exercised regularly; in some people this makes the lower face look wider and more angular.
Botulinum toxin type A temporarily reduces the contraction signal that passes from the nerve ending to the muscle. When it is injected into the masseter, the muscle's strength of contraction falls; a muscle that works less may become somewhat thinner over weeks and months. The effect is temporary: over time the nerve endings form new connections and the muscle regains its former strength. For the general properties of the medicine, see our guide “Botulinum Toxin”; this article covers only the points specific to the jaw muscle.
An important distinction: botulinum toxin does not remove the cause of the teeth-clenching habit. What it does is reduce the strength of the clenching muscle for a time. For this reason the treatment does not replace an assessment of the teeth clenching or the other measures; it is an option that may be added to them in selected people.
What is it used for?
- Jaw muscle pain, jaw tiredness in the morning and tension associated with teeth clenching and grinding (bruxism)
- A wide, angular lower face caused by a prominent, thickened masseter muscle (masseter hypertrophy)
- Some forms of jaw joint and chewing muscle pain, on the assessment of the relevant specialist and in addition to other treatments
In many countries these uses may fall outside the licensed indications of the products; in the medical literature, too, treatment of the jaw muscle is mostly referred to as 'off-label use'. In medicine, off-label use does not in itself mean that a treatment is wrong; however, it does indicate that the evidence is more limited and that the decision should be made together with the physician, with informed consent. You can ask your physician about the licensing status, for this purpose, of the product that will be used on you.
What does the evidence say for teeth grinding (bruxism)?
Teeth clenching can happen while awake or during sleep and is often associated with stress, anxiety and sleep problems. According to the patient information of the UK National Health Service (NHS), teeth grinding does not always need treatment; seeing a dentist is recommended if there is tooth damage or pain in the jaw, face or ear, and the dentist may recommend a guard worn at night to protect the teeth. On the NHS page, botulinum toxin is not listed among the standard treatments.
There are randomised controlled trials of botulinum toxin, but most are small. A 2023 meta-analysis that pooled these trials reported a reduction in bite force and in pain intensity after treatment of the jaw muscle. According to the same study, the effect appears in less than four weeks, peaks between the fifth and eighth weeks and may last up to about twenty-four weeks. A notable finding is that, although the effect of a night guard begins later, it lasts longer, particularly after the ninth to twelfth weeks.
An overview published in 2025, which assessed 14 systematic reviews on the subject together, paints a cautious picture: although most of the reviews report a reduction in pain, in the frequency of teeth-clenching episodes and in bite force, the methodological reliability of all the reviews examined was found to be 'low' or 'critically low'. The authors therefore state that no firm conclusion can be drawn and that well-designed studies are needed.
In summary: there is promising evidence, but of limited reliability, that it may provide benefit in the short and medium term for muscle pain related to teeth clenching. The long-term results of repeated treatments, and which people benefit most, are not sufficiently known.
What does the evidence say for slimming the lower face contour?
For this purpose the level of evidence is lower. A Cochrane review (2013) could not find a single randomised or controlled clinical trial comparing botulinum toxin with placebo or no treatment in bilateral benign masseter enlargement, and drew attention to the absence of high-level evidence. A 2022 review reports that, on the basis of studies that were mostly observational, a reduction in muscle volume and in the width of the lower face has been reported, but that there is no standard guideline and there are large differences between studies.
In other words, there are clinical observations showing that the treatment can thin the muscle, but these observations have not been tested in large placebo-controlled trials. How noticeable the result will be and how long it will last vary from person to person and cannot be stated with certainty in advance.
Why are an assessment by a dentist and of the jaw joint important?
- Whether teeth clenching is causing wear, fractures or damage to fillings and crowns can only be seen on a dental examination; botulinum toxin does not protect the teeth as a night guard does.
- If there is jaw pain, locking, limited mouth opening or noise from the joint, the problem may not come from the muscle alone; an assessment of the jaw joint is needed.
- A swelling that is one-sided, new, fast-growing or hard may not be masseter enlargement. An examination, and imaging if necessary, is needed to rule out conditions arising from the salivary gland and other tissues.
- Teeth clenching during sleep may be accompanied by snoring and sleep apnoea; in addition, some medicines can increase teeth clenching. Reviewing these is part of the treatment.
- Measures such as stress management, a regular sleep pattern and cutting down on caffeine and alcohol, together with a night guard, are the first step for most people.
Who may it be suitable for?
It may be considered for adults who have had a dental assessment, whose muscle pain or tiredness related to teeth clenching affects their daily life and who have not benefited enough from other measures or find it difficult to use a night guard. For the lower face contour, it may be considered in adults with realistic expectations in whom an examination has shown that the width truly comes from the muscle.
Width of the lower face does not always come from the muscle: bone structure, fat tissue and skin laxity also play a part. In these situations the change expected from the treatment is not seen; for fullness under the chin, see our article “Double Chin (Submental Fullness)”.
Who is it not suitable for, and what should you watch out for?
- Pregnancy and breastfeeding (for details, see our article “Medical Aesthetics in Pregnancy and Breastfeeding”)
- Diseases that affect muscle and nerve transmission, such as myasthenia gravis
- Active infection or a flare of a skin condition in the treatment area
- A history of allergic reaction to botulinum toxin
- Use of some medicines with a muscle-relaxing effect and some antibiotics (be sure to tell your physician)
- Jaw pain, swelling or limited mouth opening whose cause has not been investigated
- People with marked hollowing of the cheeks or sagging of the lower face: when the muscle becomes thinner, this appearance may become more noticeable
- People with bone loss (osteoporosis) or a condition involving the jawbone: because of the uncertainty explained below, the decision should be discussed separately with the physician
- People who experience intense and constant worry about their appearance: in this case we recommend reading our article “Body Image and Aesthetic Treatments” first
People whose occupation or pursuits make heavy use of functions such as chewing, playing a wind instrument or speaking for long periods should discuss the possibility of temporary weakness with the physician beforehand.
Botulinum toxin in the jaw muscle does not remove the teeth-clenching habit; it reduces the strength of the clenching muscle for a time. This is why the decision should be made after an assessment by a dentist, having discussed the possible side effects on chewing and smiling and the limits of the evidence.
Dr. Hamza GemiciMedical Director, DoktorClub · Aesthetic Medicine Physician
Who should perform it, and where?
The treatment should be carried out in a licensed healthcare facility by a physician who knows facial anatomy and is trained in this area. The smile muscles, the salivary gland and branches of the facial nerve lie close to the masseter muscle; delivering the medicine to the wrong place or the wrong depth can lead to unwanted results. For the general framework on who is authorised, see our article “Who Can Perform Medical Aesthetic Treatments?”.
The product used must be licensed, genuine and stored under appropriate conditions. Botulinum toxin products of unknown origin can cause serious poisoning; we have explained this in our article “Counterfeit Products and Unlicensed Treatments”.
Preparing for the procedure
- Explain how long you have had your complaint, whether it is more noticeable in the morning or during the day, and whether you have used a night guard before.
- Share the outcome of your dental examination, and any diagnoses relating to the jaw joint, with your physician.
- Mention all the medicines and supplements you use; do not stop your prescription medicines yourself.
- Report any botulinum toxin, filler or other procedures you have had on your face before, and their dates.
- Do not plan to have it for the first time just before an important event; assessing any asymmetry takes time.
- Read the consent form without rushing; the licensing status and the expected side effects should be included in the form.
The day of the procedure, step by step
- The physician asks you to clench your teeth and assesses by hand the borders and thickness of the muscle and the difference between the two sides; photographs are usually taken for the record.
- The safe treatment area is marked on the skin and the area is cleaned with an antiseptic solution.
- Botulinum toxin is injected with a fine needle into the muscle at several points on each side. The dose and number of points are determined by the physician according to the size of the muscle and the aim.
- The needle prick may cause brief discomfort; most people do not need numbing.
- The procedure usually takes a few minutes. At the end you are given aftercare advice and details of how to reach the physician if there is a problem.
After the procedure: the first 24 hours, the first week, the first month
Most people return to their daily life the same day. Not rubbing or massaging the area on the first day is generally recommended; follow your physician's instructions on intense exercise and other restrictions. There may be mild tenderness or a small bruise at the injection points; our article “Bruising and Swelling” offers guidance on this.
In the first week or two you may notice that your jaw tires more quickly with foods that are hard or need prolonged chewing. This is part of the expected effect of the medicine and mostly eases within weeks. A change in complaints related to teeth clenching is usually noticed in the first weeks, while slimming of the face contour is noticed later, over weeks to months.
Expected side effects and those specific to this area
- Pain, tenderness, a small bruise or swelling at the injection site
- Chewing weakness: tiring when chewing hard foods, or reduced bite strength. The reduction in bite force is an effect that has been measured in studies.
- An asymmetrical or narrower smile: this can occur when the medicine spreads to the nearby smile muscles; it improves as the effect of the medicine wears off.
- Paradoxical bulging: part of the muscle bulging outwards when the teeth are clenched. This is uncommon; reports attribute it to the medicine being unable to spread through the whole muscle because of the tendon structure within it, and to the unaffected part contracting more strongly. After a physician's assessment, an additional treatment may be needed.
- A hollowed appearance of the cheek or mild sagging of the lower face: this may become noticeable when the muscle volume decreases, especially in people with a thin face.
- Rarely, dry mouth or headache
These muscle-related effects are temporary and improve as the effect of the medicine wears off; however, this can take weeks. The general management of asymmetry and other unwanted effects is explained in detail in our article “Side Effects after Botulinum Toxin”.
If you have a new problem with swallowing, breathing, speaking or vision, call 112 (or your local emergency number) without waiting; for the other symptoms, contact the physician who treated you immediately
- Difficulty swallowing, a choking sensation or difficulty breathing
- Impaired speech, double vision or widespread muscle weakness
- Shortness of breath, widespread itching or rapidly increasing swelling of the face and tongue (allergic reaction)
- Redness, warmth or pain at the injection site that increases over days, pus-like discharge or fever
- Chewing weakness that markedly interferes with eating, or newly limited opening and closing of the mouth
Does it affect the jawbone? The debated data
Bone renews itself according to the load placed on it. When the chewing muscles weaken, the load on the lower jawbone decreases; animal studies have shown that this leads to marked bone loss. Data in humans, however, are few and not fully consistent with one another.
A systematic review and meta-analysis published in 2024 calculated a reduction of about 6 per cent in the thickness of the outer shell of the bone (cortical bone) in some regions of the lower jaw after treatment of the jaw muscle in humans, and found no marked change in bone volume or density. This result is based on only six human studies. The authors stress that most of the studies did not examine repeated treatments and high doses, and that more rigorous studies are needed to see the long-term effects.
The clinical significance of these findings, that is, whether they have any consequence in terms of the teeth, the jaw joint or the risk of fracture, is not known. The uncertainty is not considered a definite reason to avoid the treatment; but it is a good reason to avoid unnecessarily frequent repetition and, especially for people with a condition relating to bone health, to discuss the decision in detail with the physician.
When is the result seen, and how long does it last?
In the muscles of facial expression, the effect of botulinum toxin lasts about three to four months in most people. In the jaw muscle the picture is somewhat different, because two separate outcomes are followed. The reduction in muscle strength and the relief that comes with it begin within weeks; according to the meta-analysis described above, they may last up to about twenty-four weeks. Slimming of the muscle is slower: the 2022 review reports that after a single treatment the most marked slimming is usually seen at around the third month, that the effect can last six to twelve months, and that repeat treatment may be needed to maintain it.
These durations are averages from studies; the dose, the size of the muscle, whether the teeth-clenching habit continues and individual differences all change the outcome. When the effect wears off, complaints from teeth clenching may return. Decide together with your physician, according to how your complaints are, whether repeat treatment is needed and at what interval.
Realistic expectations: what can masseter treatment not do?
- It does not remove the teeth-clenching habit or its causes.
- It does not protect the teeth against wear and fracture as a night guard does.
- It does not correct structural problems in the jaw joint.
- It does not change facial width that is due to bone structure, fat tissue or skin laxity.
- Its effect is temporary; slimming of the face contour reverses over time once treatment is stopped.
- It does not make the two sides exactly equal; slight differences may remain.
Alternatives
| Option | What it does | Point to note |
|---|---|---|
| Night guard (made by a dentist) | Protects the teeth from wear; may reduce the load on the muscle | Needs regular use; does not stop teeth clenching |
| Stress management and a regular sleep pattern | Target the factors that trigger teeth clenching | The effect is slow; needs to be kept up |
| Physiotherapy and jaw exercises | May help with muscle tension and pain | Should be done under the guidance of the relevant specialist |
| Reviewing triggers | Reduces the effect of caffeine, alcohol, smoking and some medicines | Changes to medication are made only on a physician's decision |
| Masseter botulinum toxin | Temporarily reduces muscle strength and, in some people, pain; may thin the muscle | Limited evidence; side effects relating to chewing and smiling; needs repeating |
| Having no procedure | If the complaint is mild, monitoring may be enough | Dental check-ups should continue because of possible tooth damage |
If you are wondering how botulinum toxin differs from filler, see our article “Botulinum Toxin or Filler?”.
This page is for general information. The cause of your teeth-clenching complaint and the approach that is right for you can only be determined by an examination by a dentist and a physician.
Aftercare timeline
Treatment day
Do not rub or massage the area; follow your physician's instructions on intense exercise and other restrictions. If you have a problem with swallowing, breathing, speaking or vision, call 112 immediately.
Day 1
There may be mild tenderness, swelling or a small bruise at the injection points; this usually resolves on its own.
Day 3
You may notice that your jaw tires more quickly when chewing hard foods; this is part of the expected effect. If it markedly interferes with eating, tell your physician.
Day 7
Bruising and swelling have usually settled. If you notice an asymmetrical smile or a bulge that appears when you clench your teeth, let your physician know.
Day 14
The reduction in muscle strength has usually become noticeable. A follow-up, if your physician recommended one; asymmetry and paradoxical bulging can be assessed more reliably at this stage.
Day 30
Assess the change in your complaints related to teeth clenching. Slimming of the face contour may not yet be complete. Continue your dental check-ups and, if one was recommended, your night guard.
Day 90
Slimming of the muscle is usually at its most noticeable at this stage. Watch whether your complaints return; assess with your physician whether repeat treatment is needed.
Questions to ask your physician
- Does my complaint really come from the jaw muscle, or could it be the jaw joint or another cause?
- Do I need a dental assessment and a trial of a night guard before this treatment?
- Is the product you will use licensed for this purpose, or is this off-label use?
- How much benefit do you expect for me, and roughly how long could this effect last?
- How likely are chewing weakness and an asymmetrical smile in my case; what is done if they occur?
- What approach do you take if paradoxical bulging occurs?
- What is known about the effect of repeated treatments on the jawbone; do I have any condition that calls for caution here?
- Given my facial structure, could a hollowed cheek or a sagging appearance develop when the muscle becomes thinner?
- On what criterion, and how soon at the earliest, would you consider repeat treatment?
Frequently asked questions
Does masseter botulinum toxin (popularly 'masseter botox') treat teeth grinding?
It does not remove the teeth-clenching habit; it temporarily reduces the strength of the clenching muscle. Studies have reported a reduction in pain and bite force, but the reliability of the reviews assessing these studies was found to be low. It does not replace a dental assessment or measures such as a night guard.
Does 'masseter botox' slim the face?
If the width of the lower face clearly comes from the muscle, the muscle may become thinner and the face contour may soften. However, there are next to no strong placebo-controlled studies for this purpose, and the result varies from person to person. If the width comes from bone, fat tissue or skin laxity, the expected change is not seen.
How long does the effect last?
The reduction in muscle strength begins within weeks and has been reported in studies to last up to about six months. Slimming of the muscle appears more slowly; one review reports that it is most marked at around the third month and can last six to twelve months. The duration varies from person to person.
Will chewing become difficult?
In the first weeks, the jaw tiring more quickly with foods that are hard or need prolonged chewing is a commonly reported effect, and it mostly eases over time. If it markedly interferes with eating, see your physician.
Will my smile change?
If the medicine spreads to the nearby smile muscles, the smile may become asymmetrical or narrower. This effect is temporary and improves as the effect of the medicine wears off; however, it can take weeks. Having the treatment carried out by a physician who knows the anatomy reduces this possibility.
What is paradoxical bulging?
It is part of the muscle bulging outwards when the teeth are clenched after the treatment. It is uncommon; reports attribute it to the medicine being unable to spread evenly through the whole muscle. If you notice it, show it to your physician; after an assessment, an additional treatment may be needed.
Does it harm the jawbone?
There is no definite answer. A meta-analysis that pooled the small number of human studies calculated a small reduction in the thickness of the outer shell of the bone, and found no marked change in bone volume or density. The clinical significance of this and the long-term effect of repeated treatments are not known.
Is this use licensed?
In many countries, treatment of the jaw muscle may fall outside the licensed indications of the products. The licensing status varies with the country and the product; it is reasonable to ask your physician about the product that will be used on you and to expect this to be included in the consent form.
Sources
- Effectiveness of Botulinum Toxin Injection on Bruxism: A Systematic Review and Meta-analysis of Randomized Controlled Trials (Chen Y et al.). Aesthetic Plast Surg 47(2):775–790 — Aesthetic Plastic Surgery, 2023
- Botulinum Toxin for Bruxism: An Overview (Coelho MS et al.). Toxins (Basel) 17(5):249 — Toxins, 2025
- Botulinum toxin for masseter hypertrophy (Fedorowicz Z, van Zuuren EJ, Schoones J). Cochrane Database Syst Rev 2013(9):CD007510 — Cochrane Database of Systematic Reviews, 2013
- Efficacy of botulinum toxin in masseter muscle hypertrophy for lower face contouring (Kundu N et al.). J Cosmet Dermatol 21(5):1849–1856 — Journal of Cosmetic Dermatology, 2022
- Adverse effect of botulinum toxin-A injections on mandibular bone: A systematic review and meta-analysis (Moussa MS, Bachour D, Komarova SV). J Oral Rehabil 51(2):404–415 — Journal of Oral Rehabilitation, 2024
- Paradoxical masseteric bulging after botulinum neurotoxin type A injection (Sandhu S, Shaefer J). BMJ Case Rep 17(5):e258568 — BMJ Case Reports, 2024
- Teeth grinding (bruxism) — NHS, 2026
Medical Aesthetics Expert Board
This article has been reviewed by the DoktorClub Medical Aesthetics Expert Board.
