There is an old story, older than most of the beauty industry's marketing claims, about a man named Procrustes who kept an inn along the road between Athens and Eleusis. He had one bed, made of iron, and a rule that every traveler who wanted a night's rest had to fit it exactly. If a guest was too short for the frame, Procrustes stretched him on a rack until his limbs reached the ends. If a guest was too tall, he simply removed the excess with an axe. The bed itself never changed size. The traveler always did and rarely survived the adjustment.

I bring this up because a fair number of patients now walk into a consultation and ask, almost verbatim, for "the jaw botox, whatever the usual amount is," meaning an injection of botulinum toxin into the masseter muscle, the treatment responsible for narrowing a wide or square lower face by shrinking the muscle that produces it. It is one of the most requested procedures in East Asian aesthetic medicine, largely because masseter hypertrophy, an enlarged chewing muscle, often built up over years of clenching, grinding, or simply a harder bite pattern is, as one 2018 safety study bluntly puts it, "a common, prominent feature in many Asian patients" (Peng & Peng, 2018). The demand for a single, standard, "usual" dose is entirely understandable. The evidence, unfortunately, keeps refusing to provide one.

What the Injection Is Actually Doing

The masseter is one of the primary muscles of mastication, running from the cheekbone down to the angle of the jaw, and in patients who clench or grind habitually, it can hypertrophy the same way any repeatedly worked muscle does, thickening, and with it, widening the visible lower third of the face. Botulinum toxin injected into the muscle works by blocking acetylcholine release at the neuromuscular junction, producing a partial, temporary chemical denervation. The muscle fibers that can no longer be stimulated as forcefully begin to atrophy over the following weeks, and the face narrows as a direct, measurable consequence of a smaller muscle, not, contrary to what the injection is often confused with, a submandibular salivary gland problem, which is a different structure entirely and treated for a different complaint.

The Size of the Bed

The most direct answer to "how many units" comes from Hong et al. (2021), who ran a randomized, double-blind, placebo-controlled dose-finding trial in 90 subjects, split across a placebo arm and four active doses of a prabotulinumtoxinA formulation — 24, 48, 72, and 96 units per side. By week 12, masseter thickness had dropped significantly compared to placebo across every active dose, and the effect was clearly dose-dependent. But the 24-unit arm turned out to be functionally too conservative to be worth the injection, while the 96-unit arm introduced a genuinely unwanted side effect: patients reported discomfort with chewing and jaw movement that the lower doses did not produce, without a proportional gain in narrowing. The authors' own conclusion was not a number. It was a range — 48 to 72 units, with reinjection recommended around the twelve-week mark as the effect wears off. Even in a single, tightly controlled trial population, the "correct" dose spanned a factor of 1.5, and the two extremes on either side of that range were both worse choices than something in between.

Cutting to Fit

If a range already complicates the idea of a standard dose, Xie et al. (2014) complicate the underlying diagnosis itself. Their study proposed a classification system for masseter hypertrophy specifically because — as the title states outright — treatment needed to be tailored, not templated. Two patients can present with an identical complaint, "my jaw looks too wide," while the actual tissue responsible for that width differs: muscle bulk in one, a wider mandibular bone angle in another, sometimes buccal fat pad prominence layered on top of either. Botulinum toxin has an effect on precisely one of those structures. Inject the standard dose into a jaw where bone, not muscle, is doing most of the work, and the patient pays for a procedure that was never going to reach the actual cause of what she came in asking about — the Procrustean solution applied to the wrong problem.

Why the Textbooks Won't Commit to a Number

Kundu et al. (2022) took this further, systematically searching the literature through September 2021 and pooling fourteen studies with at least ten patients each. Their conclusion affirmed what most practitioners already suspected clinically: botulinum toxin is a genuinely safe and effective non-surgical option for masseter hypertrophy, with peak narrowing visible around three months and effects generally holding for six to twelve months. But the review's more interesting finding is what it declined to settle — the authors state plainly that optimal unit dosing "remains unclear," precisely because study protocols vary so widely and because ethnic differences in baseline muscle mass and bite force mean that a dose calibrated on one population's average anatomy will not transfer cleanly onto another's. The bed, in other words, is not a fixed size even at the level of an entire literature review. It is closer to a set of beds, sorted roughly by traveler.

When the Fit Is Forced Anyway

Peng and Peng (2018) offer the clearest picture of what happens when the standard protocol is applied without enough attention to individual anatomy. Reviewing 2,036 treatment sessions across 680 patients from 2011 to 2016, they documented a complication profile that was overall reassuring — a temporary, expected reduction in bite force in about 30% of treatments, and everything more serious sitting well under 3%: bruising (2.5%), headache (0.58%), a limited smile (0.15%), paradoxical bulging of the muscle (0.49%), sunken cheeks from over-treatment (0.44%), and mild facial sagging (0.20%). None of these numbers are alarming on their own. What is instructive is the authors' prescription for avoiding them: staying inside a defined quadrilateral safety zone within the muscle, positioned to spare the surrounding vessels, nerves, and non-target tissue — rather than injecting by a fixed grid regardless of where an individual patient's masseter actually sits relative to her own facial anatomy. Paradoxical bulging and sunken cheeks, in particular, read like the clinical equivalent of Procrustes' rack and axe: what happens when a technique built for an average face gets forced onto a specific one that doesn't quite match it.

The Honest Answer

None of this argues against masseter botulinum toxin, which remains one of the better-evidenced, lower-risk procedures in facial aesthetics when it is used for the problem it actually treats. What the data argues against is the version of the procedure that treats 48 units, or 60, or a fixed injection grid, as a universal answer rather than a starting range to be adjusted against an actual jaw. A patient asking for "the usual dose" is, in effect, asking to lie down on Procrustes' bed and trust that it happens to be her size. Most of the time, in trained hands, it more or less is. But the studies above exist precisely because the fit is never guaranteed in advance — and the more interesting question, before any injection, is not how many units everyone else is getting, but which structure is actually making this particular jaw look the way it does.

References

Hong, J. Y., Jeong, G. J., Kwon, T.-R., Kim, J. H., Li, K., & Kim, B. J. (2021). Efficacy and safety of a novel botulinum toxin A for masseter reduction: A randomized, double-blind, placebo-controlled, optimal dose-finding study. Dermatologic Surgery, 47(1), e5–e9. https://doi.org/10.1097/dss.0000000000002475

Kundu, N., Kothari, R., Shah, N., Sandhu, S., Tripathy, D. M., Galadari, H., Gold, M. H., Goldman, M. P., Kassir, M., Schepler, H., Grabbe, S., & Goldust, M. (2022). Efficacy of botulinum toxin in masseter muscle hypertrophy for lower face contouring. Journal of Cosmetic Dermatology, 21(5), 1849–1856. https://doi.org/10.1111/jocd.14858

Peng, H.-L. P., & Peng, J.-H. (2018). Complications of botulinum toxin injection for masseter hypertrophy: Incidence rate from 2036 treatments and summary of causes and preventions. Journal of Cosmetic Dermatology, 17(1), 33–38. https://doi.org/10.1111/jocd.12473

Xie, Y., Zhou, J., Li, H., Cheng, C., Herrler, T., & Li, Q. (2014). Classification of masseter hypertrophy for tailored botulinum toxin type A treatment. Plastic and Reconstructive Surgery, 134(2), 209e–218e. https://doi.org/10.1097/prs.0000000000000371