This study will test how well a new medicine called concizumab works in the body of people with haemophilia A or B without inhibitors. The purpose is to show that concizumab can prevent bleeds in the body and is safe to use. Participants who usually only take medicine to treat bleeds (on-demand) will be placed in one of two groups. In one group participants will get study medicine from the start of the study. In the other group participants will continue with their normal medicine and get study medicine after 6 months. Which treatment the participant gets is decided by chance. Participants who usually take medicine to prevent bleeds (prophylaxis treatment) or who are already being treated with concizumab (study medicine) will receive the study medicine from the start of the study. Participants will have to inject themselves with the study medicine 1 time every day under the skin. This can be done at home. The study doctor will hand out the medicine in the form of a pen-injector. The pen-injector will contain the study medicine. The study will last for up to 8 years. The length of time the participant will be in the study depends on when they agreed to take part and when the medicine is available for purchase in their country (or 31 December 2027 at the latest). The time between visits will be approximately 4 weeks for the first 6 to 12 months depending on the group participants are in, and approximately 8 weeks for the rest of the study. If the participant attends extra visits due to the prescription medicine not being available for purchase in their country, these will be 14 weeks apart. Participants will be asked to record information in an electronic diary during the study and may also be asked to wear an activity tracker.