Masseter Assessment · Disuse Atrophy · CityLink Mall
Botulinum toxin narrows the lower face by shrinking a muscle that has been working too hard. It has no effect whatsoever on the width of the jawbone or on the fat pad in the cheek — so whether it will do anything for your face depends entirely on which of those three is making it wide.
The masseter is a chewing muscle, and like any muscle it enlarges when it is used heavily and shrinks when it is not. Botulinum toxin injected into it binds at the neuromuscular junction and prevents acetylcholine release, so the nerve signal arrives and nothing happens. The muscle is not paralysed outright at the doses used for facial slimming — it is turned down. Chewing continues, with reduced force. What matters clinically is what follows from weeks of reduced contraction: the muscle atrophies from disuse, exactly as an immobilised limb does, and the lower face narrows as it loses bulk.
This explains the timeline, which surprises patients who expect the immediacy of filler. Nothing visible happens in the first fortnight beyond a slight reduction in chewing power. Narrowing becomes apparent from around four to six weeks and continues to develop through eight to twelve. The endpoint is reached at roughly three months, and that is when the result should be judged and any adjustment considered. Assessing at two weeks and concluding the treatment failed is the most common misunderstanding about it.
It also explains the ceiling. Only muscle atrophies. If the lower face is wide because the mandibular angle flares outward, no amount of toxin will change the bone — that is a skeletal proportion and it is altered surgically or not at all. If the fullness sits forward over the cheek rather than at the angle of the jaw, the buccal fat pad is the likely cause, and fat does not respond to neuromodulation either. Feeling the area while the patient clenches separates these in under a minute and is not optional before treatment.
The effect is temporary because nerve terminals regenerate. New axonal sprouts restore transmission over four to six months, the muscle resumes normal use, and bulk returns. Many patients find that repeated treatment allows progressively longer intervals, because a muscle held small for a year of accumulated time does not immediately rebuild to its former size. That is a pattern rather than a promise, and it depends heavily on whether the underlying habit driving hypertrophy has changed.
Selection
A wide lower face has three possible causes and they can be told apart by examination in a couple of minutes. Only one of them responds to botulinum toxin, which is why the assessment matters more than the injection technique. Patients frequently arrive certain of the diagnosis, and are sometimes right.
Toxin shrinks muscle. It does not narrow bone and it does not remove fat. Most disappointing jaw slimming is a correct treatment given to the wrong anatomy.
Realistic Expectations
The masseter contributes to the contour of the lateral lower face, and removing too much of it does not simply produce a narrower face — it produces a hollow one. Patients treated repeatedly at high doses can develop a visible depression over the lateral cheek, particularly when they are thin or have already lost midface volume. Because the muscle also provides some support to the overlying soft tissue, aggressive reduction in an older face can make jowling and midface descent more obvious rather than less. Restraint here is not conservatism; it is the difference between a slimmer face and a gaunt one.
Chewing is affected, and honesty about this is owed in advance. Most patients notice reduced bite force for the first few weeks, particularly with tough or chewy food, and adapt without difficulty. At higher doses or in smaller muscles, some report fatigue when eating a long meal. Very occasionally toxin diffusing beyond the intended plane affects a neighbouring muscle and produces a temporary asymmetric smile — uncommon, self-limiting, and minimised by correct depth and placement within the posterior part of the muscle.
Where grinding or clenching is the underlying driver, the treatment addresses the consequence rather than the cause. Reducing masseter activity often relieves the jaw ache and headache that accompany bruxism, which many patients value more than the narrowing, but the habit itself continues and the muscle rebuilds when the toxin wears off. A dental opinion is worth having where there is tooth wear or joint pain. Toxin is reasonable long-term management here, provided everyone understands it is management.
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