This clinical trial is focused on studying the 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 will evaluate a new treatment called TAK-861, which is taken in the form of a tablet. The purpose of the study is to assess how effective and safe TAK-861 is in treating narcolepsy with cataplexy.

Participants in the study will be randomly assigned to receive either the TAK-861 tablet or a placebo, which looks like the TAK-861 tablet but does not contain the active medication. The study will last for a period of 12 weeks, during which participants will take the medication daily. Throughout the study, participants will have regular check-ups to monitor their health and the effects of the treatment. The main focus will be on changes in daytime sleepiness, which will be measured using a tool called the Epworth Sleepiness Scale.

In addition to monitoring sleepiness, the study will also look at other aspects of narcolepsy, such as the frequency of cataplexy episodes and overall quality of life. Participants will be asked to complete various assessments and questionnaires to provide a comprehensive understanding of how TAK-861 affects their condition. The study aims to gather valuable information that could lead to improved treatment options for people living with narcolepsy with cataplexy.