Medically reviewed by Razvan Vasilas
DRV Clinic London
Last medically reviewed: September 2026

A strong or prominent jaw is not always caused by bone structure. In some people, the masseter muscles – the powerful muscles used when chewing and clenching – contribute significantly to the width of the lower face.

For suitable patients, botulinum toxin can be used to reduce masseter activity. As the muscle becomes less active, some of its bulk may gradually reduce, creating a softer or narrower appearance through the lower face.

That is the principle behind masseter botox.

However, making the masseter smaller does not automatically make every face look better. Bone structure, skin elasticity, facial volume, existing jowls, muscle strength and overall facial proportions all matter.

For patients considering masseter botox london, assessment should therefore focus on the whole lower face – not simply on how many injections or units can be used.

At DRV Clinic, consultations take place at our Baker Street clinic in Marylebone, Central London.

Masseter Botox at a Glance

Question

Quick answer

What is treated?

The masseter muscles at the sides of the jaw

Main aesthetic aim

Reduce muscular lower-face width in suitable patients

Does it change jawbone?

No

When can contour changes appear?

Gradually over several weeks

When may the effect be most noticeable?

Published research commonly reports maximum contouring at around three months

Is it permanent?

No

Can it make jowls more noticeable?

In some patients, yes

Current DRV Clinic price

£250

DRV Clinic location

1st Floor, 136 Baker Street, London W1U 6UD

 

What Is Masseter Botox?

For anyone searching what is masseter botox, the treatment works by temporarily reducing activity in selected fibres of the masseter muscle.

The masseter sits towards the back of each side of the jaw and is one of the major muscles involved in chewing. If you place your fingers near the angle of your jaw and clench your teeth, you can usually feel it contract.

Botulinum toxin temporarily interferes with the signalling between nerves and the injected muscle fibres. As contraction becomes weaker, continued reduction in activity can lead to a decrease in muscle thickness and prominence.

This is important because masseter botox does not remove fat or alter the jawbone. It works on muscle.

A 2026 systematic review and meta-analysis of five randomised controlled trials involving 536 participants found that botulinum toxin type A was associated with reductions in masseter prominence and muscle thickness compared with placebo. The authors also noted substantial variation between studies, meaning there is still no single treatment protocol that is appropriate for every patient.

Read the 2026 masseter botulinum toxin systematic review on PubMed

What Does Masseter Botox Do?

A common search is what does masseter botox do – and there are two separate effects to understand.

First, it reduces the strength of contraction in the treated portion of the masseter.

Second, repeated or sustained reduction in muscle activity may decrease muscle bulk. If enlarged masseters are contributing to the width of the lower face, that reduction can create a slimmer appearance.

What it cannot do is equally important.

Masseter treatment does not:

  • make a wide jawbone narrower;
  • remove excess facial fat;
  • directly tighten loose skin;
  • lift established jowls;
  • replace jawline filler where structural support is genuinely required;
  • permanently alter facial anatomy.

This is why identifying the reason for a wide lower face comes before treatment.

Who May Be Suitable for Masseter Botox?

The treatment may be considered when prominent or hypertrophied masseter muscles are contributing to a broad or square lower face.

During assessment, it is useful to examine the face both at rest and while clenching.

Relevant factors include:

  • masseter size and strength;
  • asymmetry between the two sides;
  • width of the underlying jawbone;
  • lower-face shape;
  • cheek volume;
  • chin and jawline support;
  • skin quality and elasticity;
  • existing jowling;
  • previous botulinum toxin treatment;
  • the patient’s desired result.

For example, someone with substantial muscular width and good skin elasticity may experience a pleasing narrowing of the lower face.

Another patient may already have reduced facial volume and early lower-face laxity. Aggressively reducing the masseter in that situation could make jowling or hollowing more apparent.

Good patient selection matters as much as injection technique.

Can Masseter Botox Actually Slim the Jaw?

It can slim the appearance of the jaw when muscle bulk is an important contributor to lower-face width.

It does not reduce the physical width of the mandibular bone.

This distinction is often missing from dramatic masseter botox before and after photographs online. Two people can have similarly wide-looking faces for completely different anatomical reasons.

One may have large masseter muscles. Another may simply have a wider skeletal jaw.

The first may experience a noticeable contour change from muscle reduction; the second may see a much smaller difference.

Published evidence supports an effect on masseter muscle thickness and prominence in selected patients, although results and protocols vary between studies.

How Long Does Masseter Botox Take to Work?

People searching how long does masseter botox take to work are often expecting the same timeline as treatment of forehead lines. Jaw slimming behaves differently.

Muscle activity begins changing before the final facial contour becomes apparent.

The visible slimming effect develops gradually because the muscle needs time to reduce in volume. It is therefore too early to assess the final aesthetic result only a few days after treatment.

Published reviews report that contour changes develop over the following weeks and that maximum lower-face contouring after a treatment session is often seen at around three months.

Read a review of lower-face contouring with botulinum toxin on PubMed

This gradual process can be useful aesthetically. The jaw does not suddenly change shape overnight.

How Long Does Masseter Botox Last?

There is no single answer to how long does masseter botox last because both the neuromuscular effect and the visible change in muscle volume vary between patients.

The muscle gradually regains activity as the effect of botulinum toxin wears off.

Visible facial slimming can sometimes persist beyond the period of maximum muscle relaxation because muscle volume takes time to recover. A published review reported that maximal contouring is often seen around three months, with effects in the literature lasting several months, although treatment protocols and individual responses vary.

Longevity may be influenced by:

  • baseline muscle size;
  • muscle strength;
  • individual response;
  • toxin product and dose;
  • previous treatments;
  • chewing and clenching activity;
  • treatment interval.

A previous dose should not automatically be repeated simply because a certain number of months have passed. The face and masseters should be reassessed.

Does Masseter Botox Cause Jowls?

The question does masseter botox cause jowls deserves a more nuanced answer than a simple yes or no.

Masseter treatment does not directly create loose skin. However, reducing the size of a relatively large muscle changes the volume and support within the lower face.

In someone with good skin elasticity, that may simply reveal a slimmer jaw.

In someone who already has skin laxity, reduced facial volume or early jowling, decreasing masseter bulk can make those features more noticeable.

The objective should therefore not be to make the masseter as small as possible.

For a deeper explanation, see our Masseter Botox Complications guide, where we discuss jowling, hollowing, facial asymmetry, smile changes, chewing weakness and excessive lower-face slimming.

What Can Masseter Botox Gone Wrong Look Like?

Searches for masseter botox gone wrong are common because patients increasingly research complications before having cosmetic treatment.

Possible unwanted outcomes include:

Excessive facial slimming

Reducing too much muscle bulk may leave the lower face looking hollow, narrow or less balanced with the cheeks and upper face.

More noticeable jowls

Loss of muscular volume can expose pre-existing skin laxity or lower-face tissue descent.

Smile changes

Nearby muscles help control facial expression. If botulinum toxin affects structures outside the intended treatment area, temporary smile weakness or asymmetry can occur.

Chewing weakness

The masseter has an important functional role. Reducing its activity can temporarily reduce chewing strength.

Facial asymmetry

The two masseters are not always identical before treatment. Differences in anatomy, muscle strength, dose, placement and individual response can influence the final result.

Paradoxical masseter bulging

Occasionally, part of the muscle may remain relatively active while surrounding fibres have been weakened. This can produce a visible bulge during clenching.

If an aesthetic result does not look right, immediately adding more toxin is not necessarily the correct response.

Our Botox Complications and Corrections London guide explains why botulinum toxin complications require assessment rather than the same correction strategy used for hyaluronic acid filler.

Can Masseter Botox Make the Face Look Older?

Potentially – but this depends heavily on anatomy.

Facial ageing is not simply about wrinkles. It also involves changes in volume, tissue support, skin elasticity and facial proportions.

A very prominent masseter can make the lower face look disproportionately wide. Reducing it may create a softer contour.

Yet in a face that is already narrow or losing structural volume, additional reduction can produce hollowing or make existing jowls appear more prominent.

A younger-looking face is not necessarily a smaller face.

This is one of the strongest arguments for conservative treatment and individual assessment.

Masseter Botox and Teeth Grinding

Some patients seeking jaw slimming also report clenching or grinding.

The two concerns can overlap because both involve the masseter, but they should not automatically be treated as the same problem.

Botulinum toxin can reduce the contraction force of the muscle, but persistent jaw pain, dental wear, clicking, locking or other temporomandibular symptoms may have different causes and may require assessment by an appropriate dental or medical professional.

Aesthetic jaw slimming should therefore not be presented as a universal treatment for bruxism or temporomandibular disorder