Semaglutide s.c. 2.4 mg once weekly for Obesity (STEP 1 Trial)

This study will look at the change in participants' body weight from the start to the end of the study. The weight loss in participants taking semaglutide (a new medicine) will be compared to the weight loss of participants taking "dummy" medicine. In addition to taking the medicine, participants will have talks with study staff about healthy food choices, how to be more physically active and what you can do to lose weight. Participants will either get semaglutide or "dummy" medicine - which treatment participants get, is decided by chance. Participants will need to take 1 injection once a week. The study medicine is injected with a thin needle in a skin fold in the stomach, thigh or upper arm. The study has two phases: A main phase and an extension phase.The main phase will last for about 1.5 years. Participants will have 15 clinic visits and 10 phone calls with the study doctor. Extension phase: Approximately 300 participants will continue in the extension phase in the following countries only: Canada, Germany, the UK and selected sites in the US and Japan. These participants will be in the study for about 2.5 years.They will not receive treatment, but will attend another 5 follow-up visits with the study doctor.