Jaw Botox problems: chewing, smile, what to do

The masseters are the powerful muscles at the angle of the jaw, used to clench the teeth and chew. They are sometimes injected with botulinum toxin when they are very developed and make the angle of the jaw prominent. This page is part of our atlas of problems after a botulinum toxin injection, organised area by area.

What problems can affect the jaw?

Two situations may be involved. The first is weaker chewing or a jaw that tires quickly: in a manufacturer-funded phase 3 trial, it was the most common treatment-related side effect, in 3.5% of treated people compared with none on placebo⁷, and a single-centre series recorded it after 30% of sessions⁸. It is covered in the page on difficulty chewing after a masseter injection. The second is a changed smile: this page belongs to the smile area, but it is also listed here, because an injection in the jaw region may be involved.

This needs saying clearly: apart from chewing, the adverse effects of masseter injections remain poorly documented. A 2013 Cochrane review, which searched for controlled trials of botulinum toxin for enlarged masseters, did not include any¹. A press release from the manufacturer of a type A toxin announces two phase 3 trials in this indication and reports a safety profile consistent with established uses, without detailing adverse effects specific to the jaw⁴. What is known in general applies: the effect of the toxin is temporary and wears off as the nerve terminals recover³.

Which signs mean you should see a doctor quickly?

Slightly weaker chewing or a slightly different smile is bothersome but not immediately dangerous. See a doctor promptly if chewing is so difficult that you can no longer eat normally, if your mouth droops on one side, or if you have trouble speaking clearly.

Any difficulty swallowing, speaking or breathing means calling emergency services (15 or 112 in France, or your local emergency number): the labels describe these as the rare signs of the toxin spreading away from the injection site, which require immediate medical care^2,3. Difficulty breathing or swallowing is serious and can be fatal².

Before seeing a doctor, write down the date of the injection and the day the problem appeared. If possible, contact the doctor who performed the injection.

Why are the masseters injected?

By relaxing the masseters, the injection aims to reduce their prominence at the angle of the jaw, the indication studied in the trials announced by the manufacturer⁴. It is an off-label use in France: no label provides an aesthetic indication for this area^2,3. The short-acting botulinum toxin (serotype E) is approved only for frown lines, as of 9 October 2026 in the European Union and Canada^5,6. The level of evidence is low: the Cochrane review concluded that no controlled trial had assessed the effectiveness and safety of these injections¹. In the United States, an application in this indication was accepted for review, according to the manufacturer, without being approved to date⁴. To understand why and how the masseters can be treated, see the page for this area in our atlas of treatment areas.

If your problem affects the lower face instead, see the page on the chin after an injection; if it mainly affects the way you smile, the page on the asymmetric or changed smile.