A Study of TAK-861 for the Treatment of Selected Central Hypersomnia Conditions

Inclusion criteria:

1\. Participant with a diagnosis of NT1 who has completed a controlled trial with TAK-861, and for whom the investigator has no clinical objection to their enrollment.

Exclusion criteria:

1. Participant has a treatment-emergent adverse event (TEAE) that remains severe at the time of rollover related to the trial intervention from the parent trial or discontinued because of TEAEs in the parent trial.
2. Participant has a risk of suicide according to endorsement of item 4 or 5 on the Columbia Suicide Severity Rating Scale (C-SSRS).
3. The participant has alanine aminotransferase (ALT) and aspartate aminotransferase (AST) values greater than (\>) 1.5 times the upper limit of normal (ULN).
4. Participant has a current medical disorder, other than narcolepsy with or without cataplexy, associated with excessive daytime sleepiness (EDS).
5. Participant has current active major depressive episode (MDE) or has had an active MDE in the past 6 months.
6. Participant has developed (within the last 6 months) gastrointestinal disease that is expected to influence the absorption of drugs.
7. Participant has epilepsy or history of seizure.
8. Participant has any other medical condition, such as anxiety, depression, heart disease, or significant hepatic, pulmonary, or renal disease, that requires them to take excluded medications.
9. Participant has a history of cerebral ischemia, transient ischemic attack (less than (\<) 5 years ago), or cerebral hemorrhage.
10. Participant has a history of myocardial infarction, clinically significant coronary artery disease, clinically significant angina, clinically significant cardiac rhythm abnormality, or heart failure.
11. Participant has a history of cancer in the past 5 years.