A Study of TAK-994 in Adults With Narcolepsy

Adults with narcolepsy who have completed the TAK-994-1501 study will be able to take part in this study. The main aim of this study is to check if participants have side effects from TAK-994. Participants will take one of 3 different TAK-994 dose for 8 weeks. Then, half the participants will continue with their dose of TAK-994 and half will take a placebo. In this study, a placebo will look like a TAK-994 tablet but will not have any medicine in it. Participants will take TAK-994 or placebo for 4 weeks. Participants will visit the clinic for a final check-up 2 weeks after their last dose of TAK-994 or placebo. The study doctors will check for side effects from TAK-994 and placebo throughout the study. Participants will continue to record any narcolepsy symptoms as they did in Part B of the TAK 994-1501 study.