Difficulty chewing after Botox: why it happens, how long it lasts

The masseters are the powerful muscles at the angle of the jaw, used to clench the teeth and chew¹⁰. When they are very developed, some people have them injected with botulinum toxin to slim the lower face. In France, no label provides for this area: it is an off-label use, for every toxin^6,7. A muscle relaxed to reduce its volume also loses some of its strength, which can make chewing less efficient. This is one of the jaw problems after a masseter injection. Here are the answers to the seven questions people ask most often.

Why is chewing difficult?

Botulinum toxin blocks transmission between the nerve and the injected muscle. In the masseter, this relaxation is exactly what is sought, since over time it reduces the volume of the muscle and the width of the lower face^1,10. But a weakened masseter also clenches less strongly: chewing takes more effort, the jaw tires more quickly, and discomfort or pain in the jaw may appear.

Trials group these complaints under the term "mastication disorder". In the phase 3 trial published in 2026, funded by the manufacturer of the toxin studied, it covered masseter and jaw fatigue or pain, and weakness when chewing¹. In the phase 2b trial, also funded by the manufacturer, it was described as difficulty chewing, needing to chew more or chewing fatigue².

It is the most common treatment-related side effect in the phase 3 trial: 3.5% of treated people, compared with none on placebo, and 4.2% counting all chewing problems that occurred during the trial¹. In the phase 2b trial, 2.9% of treated people were affected, slightly more often at the higher dose (4.1%) than at the lower one (1.9%)². A retrospective single-centre series gives a much higher figure: a temporary decrease in chewing strength was reported after 611 of 2,036 sessions, or 30%³. This series noted complaints at every session during follow-up³, which is not comparable with the events reported in trials.

How soon after the injection does it appear?

It appears as the effect of the toxin sets in. A review of 36 articles on the side effects of masseter injections states that most of them appear within 2 to 4 weeks of the injection⁴. A study that measured maximal bite force found it clearly reduced 3 months after an injection⁵.

How long does it last?

Usually a few weeks. In the phase 2b trial, all chewing problems resolved within a month of their onset². In the phase 3 trial, treatment-related side effects were generally mild and resolved spontaneously¹. The review of 36 articles states that most side effects of masseter injections disappear within 12 weeks⁴.

Measured bite force recovers more slowly: in a study of 20 treated people, compared with 12 untreated people, it was clearly reduced at 3 months and no longer significantly reduced at 6 months⁵. This study was partly funded by the manufacturer of the toxin used⁵. To understand what the duration of action depends on, see the page on how long the effect of botulinum toxin lasts.

Is it reversible?

Yes. The authors of the bite force study describe the reduction as reversible⁵. As everywhere, the effect of botulinum toxin is temporary: transmission between nerve and muscle gradually recovers, first through small nerve sprouts, then through the return of the original nerve terminal⁸.

Is there a treatment?

No product reverses the effect of the toxin: the problem goes away on its own over time, usually within a few weeks^1,2. In the meantime, adapting what you eat may help.

If you are considering another session, tell the doctor about this problem. In the phase 2b trial, it was slightly more common at the higher dose², and the single-centre series recommends keeping injection points within a safety zone of the muscle, away from its borders, to limit complications³.

What should you avoid?

• Do not ask for an extra injection to "compensate": muscle weakness may be made worse by another toxin injected before the effect of the previous one has worn off^6,7.
• Do not force yourself to eat hard or very fibrous food, or chew gum, while your jaw tires quickly.
• Do not confuse a tired jaw with difficulty swallowing: the latter is a warning sign that requires immediate medical attention⁷.

When should you see a doctor?

Talk to the doctor who performed the injection if the problem stops you from eating normally, if it comes with significant pain or if it does not improve. Also mention a smile that seems weaker or different: in the phase 2b trial, a few people reported a weak or altered smile². The page on a changed smile after an injection covers this situation.

More rarely, the toxin can act at a distance from the injection site. The labels then describe difficulty swallowing, speaking or breathing, which requires immediate medical attention^6,7. A series of three cases published in 2025 reports one after a masseter injection, with speech and swallowing problems starting 3 days later⁹. These signs are covered on the page about difficulty swallowing after an injection.