This study tests a drug called TAK-861 for people with narcolepsy type 1 (NT1). Narcolepsy is a condition that makes people feel very sleepy during the day and can cause sudden muscle weakness, called cataplexy. The study checks if TAK-861 is safe and helps with these symptoms. Participants in previous studies with TAK-861 can join. They must have completed a prior controlled study and not have any severe side effects from the drug. The study plans to include up to 500 people worldwide and can last up to 5 years. Participants will have to visit the clinic several times, sometimes at home, and will receive a follow-up check 4 weeks after their last dose. People with certain health conditions, like epilepsy or heart issues, or those at risk of suicide, cannot join.

• The study lasts up to 5 years with multiple clinic visits.
• Only people with NT1 from previous TAK-861 studies can join.
• Participants will only receive TAK-861, not a placebo.