Hypertrophy of the masseter is commonly seen in patients who grind their teeth (either consciously or when they sleep) and in East Asians. Bulky masseter squares the face, giving patients a more masculine/angular jawline. Muscular hypertrophy may also cause secondary bony enlargement of the mandibular angle over time due to the muscle tone.
Depending on severity, 30-50 units of botulinum toxin are required per side, with 3-5 injection points on the maximum bulk of each muscle. These points must be at least 1cm away from the anterior and posterior borders of the masseter. The injections must be deep into the muscle (almost supraperiosteally)
Overdosing the masseter could cause difficulty chewing, and if injected too close to the borders of the muscle, the toxin can spread to surrounding muscles leading to smile distortion.