This study tests whether BI 409306 prevents patients with a specific type of mental illness (attenuated psychosis syndrome) from becoming worse. This study looks at how well patients tolerate the medicine and how effective it is over 1 year.

You will be assigned by chance, like the flip of a coin, to one of the following study drug groups: • Group 1 will get BI 409306, one 50 mg tablet taken two times per day • Group 2 will get one placebo tablet taken two times a day You will have an equal chance of being placed into one of the study drug groups. BI 409306 and placebo are referred to in this consent form as the “study drug(s).” No one (including you and the study team) will know what study drug you will get. Your study doctor will be able to find out what study drug you are assigned in case of an emergency. During the study drug period, you will take one tablet of the study drug two times a day with or without food at about the same time each morning and evening (about 12 hours apart). The study drug should not be chewed or crushed. If a dose is missed by more than 6 hours, that dose should be skipped and the next dose should be taken as scheduled.