It has long been said that repeated masseter injections weaken the bite. In June 2024 Scientific Reports published a trial that measured ultrasound, electromyography and chewing together. The muscle got thinner and chewing performance did not change.
The masseter is the muscle at the side of the cheek that engages when the back teeth close. A thick one makes the lower face look square, and botulinum toxin reduces the force it exerts, which reduces its bulk.
The researchers enrolled 26 women with a thick masseter and injected 75 units of abobotulinumtoxinA per masseter. Three months later they randomised the group, giving saline to one half and a second injection to the other.
Eleven received saline and 12 received the second injection. Neither the patients nor the injectors nor the assessors knew who was in which arm. This is called triple blinding.
Measurements were taken at one, three and six months: muscle thickness on ultrasound, electrical activity in the muscle on electromyography, and how finely participants actually broke down a test material when chewing. Twenty-three completed the trial.
The masseter in the twice-injected group was significantly thinner than in the once-injected group. Electrical activity measured by electromyography was also lower in the twice-injected group.
Electrical activity is the signal picked up as the muscle engages. A lower value means the muscle was recruited less.
Masticatory performance, by contrast, did not change significantly in either group. The test measures how finely a set material is broken down by chewing, and here the two groups did not separate.
The muscle thinned and the electrical signal dropped, yet chewing held. The masseter is not the only muscle involved. The temporalis and others engage alongside it and take up part of the load, at least to a point.
This trial used abobotulinumtoxinA, 75 units per side. That number should not be set next to the units quoted for a different product.
Botulinum toxin units are defined per product. One unit of one product is not the same quantity as one unit of another. Products commonly used for the masseter in Korea are dosed in onabotulinumtoxin units, which do not convert one to one with abobotulinumtoxin units.
Clinics are sometimes compared on the number of units alone. The number only means something once the product it belongs to is known.
Twenty-three participants completed. The finding that chewing performance did not change also comes from that number, so a small difference could have gone undetected.
The two arms separated three months after the first injection, so up to that point both were under identical conditions.
What the second injection added in this trial was muscle thickness and electrical activity. That supports repeating if the aim is to reduce a square contour further.
Chewing performance held through six months. Beyond three or more repeats this trial has nothing to say.
If food starts feeling hard to chew, or hard textures become difficult, it is worth raising at the appointment. What this trial measured is laboratory performance, which is not the same as what a person notices.
Noting the product name and the units received helps when deciding the next session. With a different product the same number is a different quantity.
What this trial shows is that thinning of the muscle and weakening of the bite did not go together. The twice-injected group had a thinner muscle and lower electrical activity, while chewing performance stayed put in both groups.
That said, 23 people completed and follow-up ran to six months. It does not stretch to a conclusion that chewing strength holds indefinitely.
When planning repeat treatment it helps to record the product and the units together. Units are defined per product, so the number alone does not compare.
Masticatory performance measured at six months did not change significantly in either group. That said, 23 people completed and follow-up ran only to six months.
The masseter in the twice-injected group was significantly thinner and its electrical activity lower. It is a comparison between groups, so it does not tell an individual how many millimetres to expect.
It does not compare directly. Abobotulinumtoxin units and the units of products commonly used elsewhere do not convert one to one. The product name has to be checked alongside the number.
This trial gave two injections three months apart and followed to six months. Three or more repeats, and other intervals, were not examined.
