TAK-861 is a study drug for treating narcolepsy type 1 (NT1), a sleep disorder where people feel very sleepy during the day. This study checks if TAK-861 is safe and if it helps reduce sleepiness and sudden muscle weakness called cataplexy. The study is for people who were part of earlier TAK-861 studies. It is a long-term study, lasting up to 5 years, and will take place in many locations worldwide. Participants will visit the clinic several times, and some visits may be done at home. After the last dose of the study drug, there will be a follow-up check after 4 weeks. This study will include up to 500 participants. All participants will take the TAK-861 drug. To join, participants must have completed a previous TAK-861 study with no severe side effects. People with certain health problems, like severe liver issues or a high risk of suicide, cannot join.

• The study lasts up to 5 years with multiple clinic visits.
• Only those who completed a previous TAK-861 study can join.
• Participants must not have severe health issues or risks.