Facial lines that develop from repeated facial expression, such as forehead lines (FHL), are typically treated by selectively weakening specific muscles with small quantities of botulinum toxin. The purpose of this study is to assess adverse events and effectiveness of onabotulinumtoxinA in Japanese adults with moderate to severe FHL. Participants are placed in 1 of 4 groups, called treatment arms. Each group receives a different treatment. Participants are randomly assigned to receive onabotulinumtoxinA or placebo. There is a 1 in 5 chance that a participant will receive placebo. Around 150 adult participants with moderate to severe FHL will be enrolled in the study in approximately 15 sites in Japan. In Period 1, participants will receive intramuscular injections on Day 1. In Period 2, participants will receive up to 3 additional treatment cycles. Participants will be followed for up to 12 months. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at the study site. The effect of the treatment will be checked by medical assessments, checking for side effects and completing questionnaires.