The main purpose of this study is to evaluate the safety and efficacy of tirzepatide in adolescents that have obesity or overweight with at least one weight-related comorbidity. The study will last approximately 90 weeks and may include up to 25 visits. Participants who have completed the primary 72-week GPHP study and have been off treatment for no more than 12 weeks (including the 4-week safety follow-up period), will have the opportunity to receive an additional 156 weeks of treatment with tirzepatide as well as continuing the lifestyle intervention.