Masseter injections for teeth grinding and a square jaw: what the evidence says

The masseter: two different requests

The masseter is the thick muscle you can feel bulging at the angle of the jaw when you clench your teeth. People often speak of "botox" in the jaw when they mean treating it with botulinum toxin.

Two requests lead to it. The first is aesthetic: a thick muscle widens the lower face and gives it a square look. The second concerns bruxism, that is, clenching or grinding the teeth, especially during sleep. The two sometimes overlap, but the level of evidence is not the same.

Masseter injections: an off-label use

The product information for botulinum toxins authorised for wrinkles covers the upper face: frown lines, forehead and crow's feet. It does not cover the masseters. Injecting them into the jaw is therefore off-label use, outside the product licence. It has been practised for more than twenty years, but it is the responsibility of the doctor who offers it.

In practice, I tell you so before any treatment, and it is written in the information I give you. The examination first checks, by touch, that it really is the muscle that bulges when you clench your teeth. The decision is then yours, on the same day or later.

Square jaw: what the trials show

For a thick masseter, controlled trials now exist. A meta-analysis published in 2026 brought together five of them (536 participants): compared with placebo, the toxin reduces muscle thickness and improves the appearance of the lower face in the short term. The authors note that the ideal dose and protocol are not yet established.

The effect is not immediate: the muscle slims down gradually. In a phase 3 trial with 376 participants, funded by the manufacturer of the toxin studied, the improvement was measured at three months and was still present at six months.

Bruxism: the evidence is still weak

For clenching and grinding, the picture is less clear. An overview published in 2025 analysed fourteen systematic reviews. The toxin appeared to reduce pain, the number of bruxism episodes and bite force in most of the reviews that measured these outcomes. But all these reviews were of low or critically low methodological quality, which prevents firm conclusions.

In other words, a benefit is possible, but it has not been demonstrated with certainty. And the toxin acts on the muscle for a few months: it does not remove what drives you to clench your teeth.

Possible side effects

In the 376-participant trial, treatment-related effects were reported in 12% of treated people, compared with 3% on placebo. They were mostly mild and went away on their own:

• discomfort when chewing (fatigue, pain or reduced chewing strength), in about 3 to 4 people in 100;
• pain or discomfort at the injection site;
• a paradoxical bulging of the muscle instead of slimming, in about 2 people in 100;
• more rarely, temporary weakness of a facial muscle, which can alter the smile, in fewer than 1 person in 100. This is why injections stay behind the small smile muscle that lies in front of the masseter.

The question of the jawbone

One point is debated, and I prefer to tell you about it. In animals, toxin injected into the jaw muscles leads to clear bone loss, notably at the joint. In humans, studies are few: a meta-analysis published in 2024, which brought together six of them, found a decrease of about 6% in the thickness of the outer layer of the jawbone, with no obvious change in its volume or density.

The effects of sessions repeated over many years remain poorly known. Without being alarmist, this is one more reason not to repeat sessions without need, and to respect at least three months between two sessions, as the product information requires.

When to see a dentist first

Bruxism is first a matter for the dentist. A night guard mainly serves to protect the teeth: a Cochrane review did not show that it reduces bruxism itself, but suggests a possible benefit on tooth wear. We talk it through together, and your dentist's opinion is useful, before the toxin or alongside it, if:

• your teeth are wearing down or cracking, or a filling breaks;
• your jaw is painful, clicks or locks;
• someone hears you grinding your teeth at night;
• you have never had this problem examined by a dentist.