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

Inclusion criteria * Meet diagnostic criteria for attenuated psychosis syndrome as defined in DSM-5 and determined by SIPS administered at screening and diagnosis confirmed by NeuroCog Trials after review of video-taped SIPS interview. * Age ≥16 and ≤ 30 years at the time of consent/assent. * Male or female patients willing to use highly effective methods of contraception. * Female patients of childbearing potential must be ready and able to use highly effective methods of birth control per ICH M3 (R2) that result in a low failure rate of less than 1% per year when used consistently and correctly. Patients must agree to use birth control throughout the trial and for at least 28 days after treatment has ended. Acceptable methods of birth control include combined estrogen-progestin oral, intravaginal or transdermal contraceptives, progestogen-only oral, injectable or implantable contraceptives, intrauterine devices (IUDs), intrauterine hormone releasing systems (IUSs), bilateral tubal occlusion, vasectomized sexual partner, and complete sexual abstinence (if acceptable by local health authorities) is allowed when this is in line with the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptom-thermal, post-ovulation methods) and withdrawal are not acceptable methods of contraception. * Male patients who are able to father a child must be ready and able to be abstinent or use adequate contraception for the duration of study participation and for at least 28 days after treatment has ended. * Signed and dated written informed consent in accordance with Good Clinical Practice (GCP) and local legislation prior to any study-related procedures OR signed and dated informed consent provided by the patient's parent(s) (or legal guardian) and assent by the patient prior to any study-related procedures in accordance with GCP and local legislation. If the patient has a legal representative, then this legal representative must give written informed consent as well. Exclusion criteria * Present or past diagnosis of schizophrenia, schizophreniform, schizoaffective disorder, bipolar disorder I, major depressive disorder with psychotic features, delusional disorder, brief psychotic disorder, other specified schizophrenia spectrum and other psychotic disorder (except attenuated psychosis syndrome), and unspecified schizophrenia spectrum and other psychotic disorder, according to DSM-5. * Patients taking antipsychotic medication for less than 8 weeks, or patients taking antipsychotic medication for a longer duration but who have not been on a stable dose for 8 weeks prior to informed consent. * Patients who begin taking an antipsychotic between Visit 1 and Visit 2. * Patients who have discontinued an antipsychotic medication less than two weeks prior to randomization. * Patients taking Clozapine. * Suicidal behavior in the past 2 years reported in the Columbia Suicide Severity Rating Scale (C-SSRS) with a lethality of attempt ≥1, or with a lethality of 0 but a potential lethality of 2, or that in the judgement of the investigator would jeopardize the patient's safety while participating in the trial. The investigator/qualified rater must review all screening C-SSRS reports prior to randomization, documenting an additional interview assessing lethality of the behavior history when appropriate. * Any suicidal ideation of type 4 or 5 in the Columbia Suicide Severity Rating Scale (CSSRS) in the past 3 months (i.e. active suicidal thought with intent but without specific plan, or active suicidal thought with plan and intent). * In the judgment of the investigator, any clinically significant finding from the physical examination or laboratory value deviating from normal or any evidence of a clinically significant concomitant disease or any other clinical condition that would jeopardize a patient's safety while participating in the clinical trial. * Known diseases of the central nervous system (including but not limited to any kind of seizures or stroke). * History of significant head injury (\>5 minutes without consciousness). * A serious developmental disorder that in the judgement of the investigator would inhibit the patient's ability to comply with all study procedures, or mental retardation (documented IQ \<70), or acute attenuated symptoms exclusively related to intoxication from a psychotropic substance. * Any documented active or suspected malignancy or history of malignancy within 5 years prior to screening, except appropriately treated basal cell carcinoma of the skin or in situ carcinoma of uterine cervix. * Planned elective surgery requiring general anesthesia, or hospitalization for more than 1 day during the study period. * Meets criteria for Substance Use Disorder (DSM-5) within the six months prior to informed consent/assent. * Patients who must or wish to continue the intake of restricted medications or any drug considered likely to interfere with the safe conduct of the trial. * Patients taking strong or moderate CYP1A2 inhibitors who are also a CYP2C19 Poor Metabolizer (PM). Patients taking medication known to be a strong or moderate inhibitor of CYP1A2 must be prospectively genotyped to ensure they are not poor metabolizers of CYP2C19. (A list of CYP1A2 and CYP2C19 inhibitors can be found in the ISF.). * Patients taking strong or moderate CYP1A2 inhibitors who are also taking concomitant strong or moderate CYP2C19 inhibitors. (A list of CYP1A2 and CYP2C19 inhibitors can be found in the ISF.) * Patients with a history of moderate to severe hepatic impairment (Child-Pugh B / C). * Patients with a history of moderate to severe renal impairment (Stage 3 - 5). * Women who are pregnant, nursing, or who plan to become pregnant while in the trial. * In the judgment of the investigator, inability of the patient to comply with the clinical trial procedures. * Currently enrolled in another investigational device or drug study, or less than 30 days since ending another investigational device or drug study(s), or receiving other investigational treatment(s). * Previous participation in any BI 409306 study. * Further exclusion criteria apply