A Study of TAK-994 in Adults With Type 1 and Type 2 Narcolepsy

The drug being tested in this study is called TAK-994. TAK-994 is being tested in participants with NT1 and NT2. The study will enroll up to approximately 202 participants. The study has 4 Parts: Parts A, B, C (China only) and D. Part A - Part A has 2 cohorts \[Cohorts (A1a and A1b) A2\] In both of these Cohorts, participants will be randomly assigned (by chance, like flipping a coin) in a 2:1 ratio to receive TAK-994 or placebo up to 28 days: * Part B: In Part B, participants will be randomized in 1:1:1:1 ratio in four parallel arms to receive TAK-994 Dose 1, 2 or 3 or placebo for 56 days. Depending upon their eligibility participants completing Part B of the study treatment will be enrolled to participate in an Extension study. * Part C: In Part C, participants only from China will be enrolled and randomized in a 2:1 ratio to receive TAK-994 and placebo for 56 days. * Part D: Participants will be included in two cohorts \[Cohorts (D1a and D1b) and D2\] and will be randomized in 2:1 ratio to receive TAK-994 or placebo for 28 days. The dose will be selected based on the safety and tolerability in Part A. This multi-center trial will be conducted in the United States, Japan, China, Italy, France, and European Union. The overall duration of the study is 63 days. Participants will be followed up for 7 days after the last dose of study drug.