The main purpose of this study is to find out how well and how safely tirzepatide works long-term in adults who have type 1 diabetes and obesity or overweight. Participation in the study will last about 20 months.

Inclusion Criteria:

• Have type 1 diabetes and on insulin treatment for at least one year prior to screening
• Have an HbA1c value of 7.0% to 10.5% inclusive, at screening
• Have a body mass index (BMI) of ≥25 kilograms per square meter (kg/m2) at screening
• Are of stable weight for at least 90 days prior to screening and agree to not start an intensive diet or exercise program during the study

Exclusion Criteria:

• Have experienced two or more events requiring hospitalization due to poor glucose control (hyperglycemia or (diabetic ketoacidosis (DKA)) during the period of 180 days prior to screening and until randomization.
• Have experienced one or more events of severe hypoglycemia during the period of 90 days prior to screening and until randomization.
• Are currently receiving or planning to receive treatment for diabetic retinopathy and/or macular edema
• Have had chronic or acute pancreatitis
• Have used any weight loss drugs or alternative remedies, including herbal or nutritional supplements, within 90 days prior to screening

Anonymized individual patient level data will be provided in a secure access environment upon approval of a research proposal and a signed data sharing agreement.

Data are available 6 months after the primary publication and approval of the indication studied in the US and EU, whichever is later. Data will be indefinitely available for requesting.

A research proposal must be approved by an independent review panel and researchers must sign a data sharing agreement.