The main aim is to evaluate the safety and tolerability of TAK-861 in participants with type 1 narcolepsy, who were exposed to previously tested doses of TAK-861.

The drug being tested in this trial is called TAK-861. TAK-861 is being tested to treat people who have narcolepsy type 1 (NT1). This trial will look at the safety of TAK-861 along with improvement in narcolepsy symptoms, including excessive daytime sleepiness (EDS) and number of cataplexy episodes.

This trial was initiated in parallel with the parent phase 2 trials, TAK-861-2001 (NCT05687903) and TAK-861-2002 (NCT05687916), which included participants with NT1 and narcolepsy type 2 (NT2), respectively. This long-term extension (LTE) trial enrolled participants with both diagnoses from the two phase 2 trials. As the TAK-861-2002 trial did not meet prespecified criteria, all participants with NT2 were discontinued, and moving forward this extension study only includes participants with NT1 who previously completed a parent trial. Parent trials include all TAK-861 phase 3 trials in addition to the phase 2 trial participants from TAK-861-2001.

The trial will enroll up to 500 participants. All participants in the trial will receive TAK-861. Participants who were previously on a placebo dose will be assigned to one of the TAK-861 doses randomly.

This multi-center trial will be conducted worldwide. Study period is approximately 5 years or until the study is stopped at the sponsor's discretion, or the product is approved and launched. Participants will make multiple visits to the clinic (with some visits optionally conducted by home health) and will have a follow-up assessment 4 weeks after the last dose of trial intervention.

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.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.

All participants in this long term, dose blinded study received study drug.