Verdict: only a clinic can, and it is temporary. Botulinum toxin injected into the masseter weakens the muscle, and a weakened muscle shrinks. A systematic review puts the reduction at about 26 to 31% of muscle thickness at three months. The effect fades as the muscle recovers. Nothing about the bone changes.
This article is informational. It is not a recommendation, and it does not replace an assessment by a clinician who can examine your jaw.
What it does
The masseter is the thick muscle you feel bulge at the back of your jaw when you clench. In some people it is large enough to widen the lower face, and in about 40% of masseter hypertrophy cases the enlargement is one-sided, which is why it shows up in asymmetry conversations so often.
Botulinum toxin blocks the signal from nerve to muscle. The masseter cannot contract as hard, and over the following weeks it loses bulk, the same way any muscle does when it stops being loaded. A 2022 systematic review in the Journal of Cranio-Maxillofacial Surgery pooled 28 studies with 748 patients treated for masseter hypertrophy. At three months, mean masseter thickness was down 26 to 31% with a single injection site, 28% with two, 12 to 27% with three and 22 to 30% with six. The reviewers found the studies heterogeneous and at high risk of bias, and suggested using the fewest injection sites until better evidence exists.
For scale: on a masseter that measures 12 to 14 mm, a 30% reduction is roughly 3 to 4 mm. A lifetime of one-sided chewing produces about 1 mm. This is the one method on the whole list that moves the muscle layer by a visible amount.
How long it lasts
Bite force is the cleanest way to measure the muscle effect. In a one-year study of 20 people treated for aesthetic masseter reduction, maximal bite force was significantly reduced at 3 months and no longer significantly reduced at 6 months. The authors noted the visible slimming may outlast the measurable weakness, since the muscle regains strength before it regains all of its bulk.
So the working window is months, not years. Keeping the result means repeating the treatment, and the schedule is a clinical decision, not something to copy from a forum.
What it does not change
- The mandible. The jaw angle, jaw width and chin are bone, and the injection does not reach them. If your lower face is wide because the bone is wide, the muscle can shrink and the width stays.
- Fat. Submental and jowl fat sit in front of the muscle and are unaffected. Weight loss is the lever for that layer.
- Chin offset. A chin that sits off the midline is a skeletal position. Shrinking one masseter can make the two sides of the lower face look more even in bulk; it does not move the chin.
- Skin. A face that reads soft because of skin laxity will read the same after the muscle shrinks, sometimes softer.
Things to know before a consultation
- Chewing is affected. That is the mechanism. Some people notice fatigue when eating hard foods in the first weeks.
- Asymmetric results are possible. Two muscles do not always respond equally, and the dose per side is part of what a clinician assesses.
- A smile can change. The risorius and zygomaticus muscles sit near the injection area; diffusion into them can alter the smile temporarily. Placement matters.
- Repeated treatment over years has not been studied well. The 2022 review's main complaint was the quality and inconsistency of the evidence.
- The reasons people seek it overlap with jaw clenching and grinding, which are dental questions first. If one masseter has grown over the past year, ask a dentist about bruxism before asking anyone about aesthetics.
Is the width even muscle?
Before treating the muscle, it is worth knowing how much of the width is muscle. Clench hard and press a fingertip into each jaw angle: the bulge is masseter. Relax fully and feel the corner of bone beneath. The jawline tool gives jaw width as a ratio of face width and the jaw angle in degrees, read from the bone landmarks. A wide ratio with a small clench bulge is bone, and the injection will not change the number much. A modest ratio with a large clench bulge is muscle, and it is the case the procedure was designed for.
Talk to a clinician about the rest.
Sources
Rauso R, Lo Giudice G, Tartaro G, Zerbinati N, Nicoletti GF, Fragola R. Botulinum toxin type A injections for masticatory muscles hypertrophy: a systematic review. Journal of Cranio-Maxillofacial Surgery 2022;50(1):7-18, doi:10.1016/j.jcms.2021.09.019, PubMed 34620536. Ågren M et al. Duration of bite force reduction following a single injection of botulinum toxin in the masseter muscle bilaterally: a one-year non-randomized trial. Journal of Oral Rehabilitation 2023;50(5):343-350, doi:10.1111/joor.13434, PubMed 36810787. Unilateral masseter hypertrophy and one-sided chewing: Journal of Prosthetic Dentistry 2025; Rajesh K et al., Cureus 2024;16(3):e56849; PMC11860558.