A Study of TAK-861 for the Treatment of Narcolepsy Type 1

Inclusion Criteria:

1. The participant has a body mass index (BMI) within the range 18 to 40 kilograms permeter square (kg/m^2).

2. The participant has an International Classification of Sleep Disorders, ThirdEdition (ICSD-3) or International Classification of Sleep Disorders, Third Edition,Text Revision (ICSD-3-TR) diagnosis of NT1.

3. The participant has greater than or equal to (≥)4 partial or complete episodes ofcataplexy/week (WCR).

4. The participant is positive for the human leukocyte antigen (HLA) genotypeHLA-DQB1*06:02 or results from radioimmunoassay indicate the participant'scerebrospinal fluid (CSF) orexin (OX)/hypocretin-1 concentration is less than orequal to (≤)110 picograms per milliliter (pg/mL) [or less than one-third of the meanvalues obtained in normal participants within the same standardized assay].

Exclusion Criteria:

1. The participant has a current medical disorder, other than narcolepsy withcataplexy, associated with EDS.

2. The participant: (a) has a history of myocardial infarction; (b) has a history ofclinically significant hepatic disease, thyroid disease, coronary artery disease,cardiac rhythm abnormality or heart failure; or (c) has any medical condition (suchas unstable cardiovascular, pulmonary, renal or gastrointestinal disease).

3. The participant has current or recent (within 6 months) gastrointestinal diseasethat is expected to influence the absorption of drugs.

4. The participant has a history of cancer in the past 5 years.

5. The participant has a clinically significant history of head injury or head trauma.

6. The participant has a history of epilepsy, seizure, or convulsion.

7. The participant has a history of cerebral ischemia, transient ischemic attack (<5years from screening), intracranial aneurysm, or arteriovenous malformation.