A Study to Assess BOTOX Injections for the Change of Masseter Muscle Prominence in Adult Participants

The masseter muscle is one of the muscles in the lower face used for chewing. The prominence of the masseter muscle can appear as a widened lower face which some individuals deem as aesthetically undesirable and can be treated by selectively weakening the masseter muscles with small quantities of botulinum toxin. The purpose of this study is to assess how safe and effective injection of onabotulinumtoxinA (BOTOX) is in adult participants with Masseter Muscle Prominence (MMP). BOTOX is being investigated for the treatment of Muscle Masseter Prominence. In Period 1, participants are placed in 1 of 2 groups, called treatment arms. Each group receives a different treatment, placebo or BOTOX. There is a 1 in 4 chance that participants will be assigned to placebo. In Period 2, eligible participants can receive an optional open-label retreatment of BOTOX. Approximately 200 adult participants with MMP will be enrolled in the study at approximately 20 sites in the United States. Participants will receive intramuscular injections across both the right and left masseter muscle of either BOTOX or placebo on Day 1. Participants who are eligible for re-treatment will be given a BOTOX injection on either the Day 180, 210, 240, or 270 visit and will be followed up for up to 6 months. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular monthly visits during the study at a hospital or clinic. The effect of the treatment will be checked by medical assessments, checking for side effects, and completing questionnaires.

Inclusion Criteria: * Participant meets the following criteria: * Bilateral Grade 4 or Grade 5 Masseter Muscle Prominence (MMP) (identical grades for left and right sides of the face), as determined by the investigator using the Masseter Muscle Prominence Scale (MMPS). * Bilateral Grade 4 or Grade 5 MMP (identical grades for left and right sides of the face), as determined by the participant using the Masseter Muscle Prominence Scale - Participant (MMPS-P). * Investigator and participant scoring of MMPS and MMPS-P must be the same. * Body mass index (BMI) ≤ 30 kg/m\^2 using the calculation: BMI = weight (kg)/height (m)\^2. Exclusion Criteria: * Excess lower facial fat, jowling, loose or lax skin in lower face, or parotid gland prominence that could interfere with MMPS or MMPS-P grading. * Asymmetry of left and right sides of the face that could prevent identical MMPS or MMPS-P grading on both sides of the face. * History of or current temporomandibular disorder (TMJD), or presence of signs/symptoms of possible TMJD.