One of the strongest muscles in the body relative to its size. Years of clenching make it bigger, which widens the lower face. Relaxing it reverses that — but only the part that's muscle.
Neurotoxin reduces the masseter's contraction force. That relieves clenching and grinding within one to two weeks, and the muscle shrinks from reduced use over four to six weeks, slimming the lower face. Lasts four to six months, often longer with repeat treatment. It only affects muscle — if your jaw width is skeletal, this won't change it.
Put your fingers on your jaw, just in front of the earlobe. Clench.
That's masseter. It's muscle, it responds to neurotoxin, and the bigger the bulge the more change you'll see.
The width is skeletal — the angle and flare of the mandible. Neurotoxin won't alter bone.
Most people have some of each. The muscular part responds; the skeletal part doesn't, and no dose changes that.
This is the question that determines whether you'll be happy. Someone with a genuinely enlarged masseter gets a visible change. Someone with a wide mandible and modest muscle gets very little — and that's worth knowing before paying for it rather than after six weeks of waiting.
A wide or square lower face from an enlarged masseter — often built over years of clenching without realising it. Reducing the muscle produces a softer, more tapered jawline over four to six weeks.
Jaw pain on waking, tension headaches at the temples, tooth wear. Reducing contraction force relieves the pressure on teeth and joint, usually within one to two weeks.
Most patients have both, whether or not they came in for both. Someone booking for jaw slimming frequently turns out to be grinding at night; someone booking for grinding relief usually notices their face narrowing as a side effect they didn't expect.
Worth being precise about. Neurotoxin reduces how hard you can clench. It doesn't switch off the neurological drive to do it — you'll still grind, just with less force.
That's usually enough to resolve the pain and the headaches, and to slow the tooth wear. But it doesn't replace a nightguard, which physically protects the tooth surfaces. Many patients do best with both, and if you already have a guard from your dentist, keep using it.
The most common source of confusion — patients treated for appearance sometimes conclude at week two that it hasn't worked.
Longer than facial neurotoxin, typically four to six months — the masseter is a large muscle treated at higher doses. And duration frequently extends with repeat treatment, since a muscle that's been smaller for a year takes longer to rebuild.
Standard protocols treat both sides identically. Most jaws aren't identical.
Placement too high or too far forward risks affecting the risorius and zygomaticus — the smile muscles. The result is a flattened or uneven smile. It's temporary, but it lasts as long as the treatment does, which is months. This is a mapping error rather than a normal side effect.
Excessive or overly frequent dosing produces genuine chewing weakness and, over time, hollowing at the cheek as the muscle loses substantial volume. That reads as gaunt rather than slim, and it's a real possibility — the masseter contributes to lower face structure, and taking too much away is as much a problem as taking too little.
Building the result across sessions beats an aggressive first treatment. You can always add.
Reducing the masseter narrows the lower face. It doesn't define the jawline border — those are different problems and they need different treatments.
Patients wanting a sharper, more delineated jaw usually need filler along the mandibular border alongside masseter reduction. Narrowing without defining can leave the lower face soft rather than sculpted.
Which you need depends on whether the complaint is width or lack of edge. Frequently it's both. More on the distinction.
The clench test takes ten seconds and it's the whole consultation in miniature. If nothing bulges under my fingers, the width is bone and I'll say so — that costs me the booking and saves the patient six weeks of waiting for a result that was never coming. When it does bulge, this is one of the most reliable treatments I offer.
Is my wide jaw muscle or bone?
Place your fingers on the jaw just in front of the earlobe and clench. If a firm bulge pushes out against your fingers, that is masseter muscle and neurotoxin will reduce it. If the width feels the same clenched and relaxed, the shape is skeletal — determined by the angle and width of the mandible — and neurotoxin will not change it. Many people have some of both, and the muscular portion is the part that responds.
How long does masseter Botox take to work?
Two different timelines. Reduced clenching force and jaw tension are commonly noticed within one to two weeks. Visible slimming takes considerably longer, developing over four to six weeks as the muscle reduces in bulk from being used less forcefully. Patients treated primarily for appearance should expect to wait about six weeks before assessing the result.
Will masseter Botox affect chewing?
At appropriate doses, chewing is preserved. The masseter is one of the strongest muscles in the body relative to its size, and treatment reduces force rather than eliminating function. Some patients notice fatigue with tough or chewy foods during the first two to three weeks. Marked difficulty chewing suggests over-treatment rather than a normal response.
Can masseter Botox change my smile?
It can if product is placed too high or too far forward, where it may affect the risorius or zygomaticus muscles and flatten the smile or produce asymmetry. This is a placement error rather than an expected effect, and it is temporary. It is a specific reason injection points should be mapped to the safe zone of the masseter rather than approximated.
Does masseter Botox stop teeth grinding?
It reduces the force of grinding rather than stopping the behaviour. The neurological drive to clench continues, but the muscle can no longer contract as forcefully, which relieves jaw pain and tension headaches and reduces damage to the teeth. It does not replace a nightguard, which physically protects the tooth surfaces, and many patients benefit from using both.
Can masseter Botox correct an uneven jaw?
Where the asymmetry is caused by uneven muscle bulk — commonly from chewing predominantly on one side — different doses per side can bring the two closer to balance. Where the asymmetry is skeletal, neurotoxin will not correct it and may make it more noticeable by reducing muscle on one side only. Establishing which applies is part of the assessment.
How long does it last?
Typically four to six months, which is longer than facial neurotoxin because the masseter is a large muscle treated at higher doses. Duration often extends with repeated treatment, as sustained reduction in use leads to lasting reduction in bulk and less product is required to maintain the result.
Can too much masseter Botox cause problems?
Yes. Excessive or overly frequent dosing can produce marked chewing weakness and, over time, hollowing at the cheek as the muscle loses substantial volume, which can make the face look gaunt rather than slim. Because the masseter contributes to lower face structure, taking too much away is a real risk. Building the result gradually across sessions is preferable to aggressive initial dosing.
If something bulges, this treatment works well. If nothing does, Dr. Mortazavi will tell you that at consultation rather than at week six.
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