This clinical trial is focused on studying a condition known as Narcolepsy with Cataplexy, which is also referred to as Narcolepsy Type 1. Narcolepsy is a sleep disorder that causes excessive daytime sleepiness and sudden loss of muscle control, known as cataplexy. The study is evaluating a treatment called TAK-861, which is taken in tablet form. The purpose of the study is to assess the long-term safety and tolerability of this medication in individuals with this condition.

Participants in the study will take TAK-861 over a period of time to see how well they tolerate the medication and to monitor any side effects that may occur. The study aims to ensure that the medication is safe for long-term use. Participants will be monitored for any treatment-emergent adverse events, which are any unwanted effects that occur after starting the medication.

The study will also look at changes in sleep patterns and symptoms from the beginning of the study to the end. This includes measuring how long participants can stay awake during the day and tracking any changes in their cataplexy symptoms using a diary. The study is expected to continue until early 2028, providing valuable information on the long-term use of TAK-861 for managing Narcolepsy with Cataplexy.