Key points
- An enlarged masseter muscle can make the lower face look wide, and botulinum toxin injection is a nonsurgical method considered as an alternative to surgery [1].
- After a single injection, the maximum effect usually appears at about 3 months and can last 6 to 12 months, and multiple injections may be needed to maintain it [1].
- The appropriate dose varies greatly between studies and depends on ethnicity and the degree of muscle enlargement, so it has not been established, and there are no standard guidelines [1].
- Studies on teeth grinding have suggested possible reductions in pain and grinding frequency, but methodological limitations make it difficult to draw firm conclusions [6].
What causes a square jaw?
An angular-looking jaw may be caused by the shape of the lower jawbone, or by thickening of the masseter, the muscle used to chew food. Masseter muscle hypertrophy (enlargement) makes the lower face look wide and often becomes a cosmetic concern [1].
Clenching or grinding the teeth (bruxism) results from the contraction of jaw muscles such as the masseter and the temporalis, and the literature notes that this can cause the chewing muscles to enlarge [3]. Because botulinum toxin is an injection that acts on muscle, the change you can expect is limited if bone is the main cause.
How do the injections work?
Botulinum toxin is an injection that reduces excessive muscle contraction, and a 2023 narrative review stated that when a specific muscle is overactive, such as a bulky masseter, precisely targeting that muscle can achieve cosmetic results [2]. The same review summarized injection sites and techniques separately for Asian and white patients, and also discussed the advantages of injecting under ultrasound guidance [2].
A 2022 review stressed that injection points must stay well within the borders of the masseter muscle [1]. Because the position and thickness of the muscle differ from person to person, an examination in which the muscle is felt directly and its movement checked before injection is important.
When does the effect appear, and how long does it last?
A 2022 review of studies on botulinum toxin for masseter hypertrophy selected 40 papers and compared 14 of them, those with 10 or more participants, in a table [1]. According to the review, after a single injection the maximum effect usually appears at about 3 months, and the effect can last 6 to 12 months [1].
It stated that multiple injections may be needed to maintain the effect over the long term [1]. Because this review broadly included retrospective and prospective studies, case series, case reports, and expert reviews, it combines studies with different levels of evidence [1]. The size and duration of the effect can vary from person to person depending on the thickness of the muscle and your chewing habits.
Why there is no set answer on dose and technique
There are no standard guidelines for botulinum toxin injection for masseter hypertrophy [1]. The review stated that the appropriate number of units varied greatly between studies and could not be determined because patients' ethnicity and the degree of muscle enlargement, or the way it was measured, differed [1]. It therefore concluded that the injection technique and total dose must be decided individually for each person [1].
Because the study participants were of diverse ethnicities, it is difficult to apply results from one particular group directly to other people. Even with what looks like the same square jaw, the plan can differ depending on the condition of the muscle.
When teeth grinding is also present
In addition to muscle enlargement, teeth grinding can cause tooth wear, fractures of dental restorations or teeth, and sensitive or painful teeth [3]. However, the literature explains that teeth grinding itself does not require treatment and is managed when it causes problems [3]. Management options include oral appliances that protect the teeth, biofeedback and relaxation, and better sleep hygiene, and the review stated that there is no evidence for irreversible adjustment of the bite [3].
Regarding botulinum toxin, a 2019 systematic review reported, based on 4 randomized controlled trials, that it reduced the frequency of grinding, pain, and maximum bite force [4]. In a 2025 systematic review, 6 of 12 studies reported reduced muscle activity and 3 reported reduced pain, but the differences between studies were large, so long-term follow-up studies were said to be needed [5].
A 2025 analysis that pooled 14 systematic reviews stated that the confidence in all of the reviews was low or very low, so no firm conclusions could be drawn [6]. No review properly evaluated the effect on the functional movement of the jaw, and concerns about side effects were also raised [3, 6].
What to check during your consultation
Square jaw injections may seem relatively simple, but the results you can expect depend on the extent and thickness of the muscle, the shape of the bone, and whether you grind your teeth. It is a good idea to identify the cause with the specialist who examines you in person and to discuss a maintenance plan as well.
- Whether the angular jaw is caused by muscle or by bone
- Whether you grind or clench your teeth
- The interval and number of injections needed to maintain the effect
- The possibility of discomfort such as changes in chewing strength or facial expression
Frequently asked questions
- How long does the effect of square jaw injections last?
- According to a review, the maximum effect usually appears about 3 months after injection and can last 6 to 12 months [1]. The review stated that multiple injections may be needed to maintain the effect [1]. How long it lasts varies from person to person.
- Can injections help if my jawbone protrudes?
- Botulinum toxin is a nonsurgical method used for an enlarged masseter muscle [1]. Because it acts on muscle, the change may be limited if the shape of the lower jawbone is the main cause. The cause is identified through an examination and, if needed, imaging tests.
- Can injections help if I grind my teeth?
- Several systematic reviews have reported possible reductions in pain, grinding frequency, and maximum bite force [4, 6]. However, because the confidence in these reviews was low, it was stated that firm conclusions are difficult to draw [6]. It is a good idea to discuss teeth grinding together with other management methods, such as oral appliances [3].
References
- Kundu N, Kothari R, Shah N, et al. Efficacy of botulinum toxin in masseter muscle hypertrophy for lower face contouring. J Cosmet Dermatol. 2022.PubMed 35176198DOI
- Hong SO. Cosmetic Treatment Using Botulinum Toxin in the Oral and Maxillofacial Area: A Narrative Review of Esthetic Techniques. Toxins (Basel). 2023.PubMed 36828397DOI
- Beddis H, Pemberton M, Davies S. Sleep bruxism: an overview for clinicians. Br Dent J. 2018.PubMed 30237554DOI
- Fernández-Núñez T, Amghar-Maach S, Gay-Escoda C. Efficacy of botulinum toxin in the treatment of bruxism: Systematic review. Med Oral Patol Oral Cir Bucal. 2019.PubMed 31246937DOI
- Yacoub S, Ons G, Khemiss M. Efficacy of botulinum toxin type A in bruxism management: A systematic review. Dent Med Probl. 2025.PubMed 40035138DOI
- Coelho MS, Oliveira JMD, Polmann H, et al. Botulinum Toxin for Bruxism: An Overview. Toxins (Basel). 2025.PubMed 40423331DOI
This article is an English translation of general health information that DOY JUN Plastic Surgery compiled in Korean from the summaries (abstracts) of papers in international academic journals. Treatment methods and results vary depending on each person's condition, so please consult a specialist for an accurate diagnosis and treatment plan.