Masseter Botox: jaw muscle injections, what is known

The jaw is one of the eleven areas of our "why and how" atlas of botulinum toxin, added because the question comes up often. This page explains the role of the masseter, what is known about treating it with botulinum toxin, why it is an off-label use in France, and what its limits are. It does not replace a consultation.

Why talk about the masseter?

The masseter is a powerful muscle on each side of the face, between the cheekbone and the angle of the jaw. It is used to clench the teeth and to chew. When it is very developed, this is called masseter hypertrophy or prominence: it forms a bulge near the angle of the jaw and widens the lower face^1,3.

Botulinum toxin relaxes the injected muscle; in the masseter, this relaxation also leads to a gradual reduction in its volume, which explains the reduction in width observed in some trials^1,3. A phase 3 trial published in 2026, funded by the manufacturer of the toxin studied, measured a mean reduction in lower face width of about 5 mm on day 90, compared with no change on placebo³.

What the toxin does not do: it does not change the jaw bone and does not act on sagging skin or fat in the lower face. It is not a guaranteed way to change the shape of the face: results vary widely from one person and one study to another².

When is the toxin used?

There are no official criteria in France, since no label provides for this area. Recent trials concern adults whose masseter is rated "marked" or "very marked" on an assessment scale³. The 2013 Cochrane review noted that benign masseter hypertrophy is an uncommon condition of uncertain cause; the trials it looked for had to concern hypertrophy confirmed by clinical and radiological examination¹.

The contraindications in the labels apply whatever the area: allergy to the toxin or to a component of the product, a generalised disorder of the muscles and of the junction between nerve and muscle (myasthenia gravis, Lambert-Eaton syndrome), an infection at the injection site^5,6. Treatment is not recommended during pregnancy or breastfeeding^5,6.

Which toxin for the masseter?

The type A toxins approved for aesthetic use in France cover frown lines and, depending on the product, crow’s feet and forehead lines^6,7. None provides for the masseter. The short-acting botulinum toxin (serotype E) is approved for frown lines only, in the European Union and in Canada^5,8. For this area, any injection is therefore off-label in France. In the United States, the manufacturer of a type A toxin announced on 4 August 2026 that its application for this area had been accepted for review by the FDA; at the date of that press release, the use was therefore not approved there⁴.

Is it right for me?

Your doctor will first try to find out whether the width of your jaw is really due to the muscle, rather than to the bone or the tissues of the cheek. They will ask whether you clench your teeth or have jaw pain, and will take into account your medical history, your treatments and any pregnancy plans.

They will also explain the limits of the data: few trials, variable results, no optimal protocol defined². The neighbouring areas are covered on the pages about the chin and about the neck and platysma bands.

Questions to ask your doctor: is this treatment off-label? Is my jaw wide because of the muscle? What result is reasonable, and after how long? Could my chewing be affected? What happens if I stop?

Onset and duration

No label gives an onset or duration for this area. In the 2026 phase 3 trial, 51.2% of treated people had an improvement of at least two grades on day 90, compared with 2.2% on placebo, and the improvement was maintained up to day 180³. The 2026 meta-analysis already observes a benefit at week 4, but stresses the need for trials with longer follow-up².

How onset works in general is explained on the page about how long botulinum toxin takes to work.

What are the risks in this area?

In the 2026 phase 3 trial, most side effects were mild, non-serious and resolved³. The most common treatment-related side effect was a chewing problem (jaw fatigue or pain, weaker chewing), in 3.5% of treated people compared with none on placebo³. The injection-site reactions described in the labels (pain, swelling, small bruise) are possible⁵. Since the masseter is used for chewing, this is a question to ask your doctor before the injection; the page on difficulty chewing after a masseter injection explains how long it lasts and what to do.

As with any botulinum toxin injection, difficulty swallowing, speaking or breathing requires immediate medical attention⁵. If something unexpected happens after a jaw injection, the page on the jaw after an injection helps you find your way.