Treatment of masseter muscle hypertrophy with botulinum toxin type A injection: A review of adverse events

Abstract Background The popularity of noninvasive botulinum toxin type A (BTX‐A) injections for masseter muscle hypertrophy is increasing among Asian individuals with a square‐shaped lower face. Aims This study aimed to analyze the adverse events (AEs) caused by BTX‐A injections into the masseter muscle. Patients/Methods This observational study retrospectively evaluated 46 250 patients who underwent BTX‐A injections into the masseter muscle in 2022. The inclusion criteria were the diagnosis of an AE by the physician at the return visit and subsequent follow‐up of progress ( n = 223). The patients who were lost to follow‐up ( n = 40) were excluded from the study. Results Among the 223 patients with AEs, the most common AE was paradoxical bulging (88.3%, n = 197/223). The average period from treatment until confirmation of improvement was 159.6 ± 113.6 days (range 13–667 days) for all AEs, all of which were temporary. The period until improvement was 166.1 days in the intervention group ( n = 122) and 151.9 days in the observation group ( n = 101) ( p = 0.24). As the period until improvement of AEs included the period until the patients visited the clinics and the improvements were confirmed by physicians, the actual period was likely to have been shorter. Conclusions (1) All AEs were temporary. (2) All AEs improved within 22.2 months (within 5.3 ± 3.8 months on average). (3) There was no significant difference between the intervention and observation groups in the period until the improvement of AEs.