A Study to Test How BI 3031185 is Tolerated by People With Borderline Personality Disorder or Attention-deficit/Hyperactivity Disorder

This study is open to adults with certain mental health conditions. The purpose of this study is to find out how a medicine called BI 3031185 is tolerated by people with certain mental health conditions. Participants are put into 2 groups of equal size randomly, which means by chance. Group 1 takes a single dose of BI 3031185 and Group 2 takes placebo. After a 2-week break, Group 1 takes placebo and Group 2 takes a single dose of BI 3031185. Participants take BI 3031185 and placebo as tablets. Participants are in the study for about 3 months. They visit the study site 6 times and have 3 phone or video call visits. For 2 of the visits, participants stay overnight at the study site for 2 nights. During all the visits, doctors check participants' health and take note of any unwanted effects.

Inclusion Criteria: * Male, female, and non-binary participants, 18 to 45 years of age, both inclusively, at the time of consent * Meet current diagnostic and statistical manual of mental disorders, fifth edition (DSM-5) criteria as primary diagnosis as assessed by the mini international neuropsychiatric interview (MINI) at screening for borderline personality disorder (BPD) OR attention-deficit/hyperactivity disorder (ADHD). With full implementation of version 5.0 of this clinical trial protocol (CTP), only participants with a primary diagnosis of ADHD as assessed by the MINI will be included in the trial. * Willingness to abstain from alcohol for 24 h, and all other drugs of abuse including cannabis for 72 h prior to Visits 2 and 3 (Day -1). Willingness to abstain from alcohol and cannabis for 72 h after investigational medicinal product (IMP) administration, as well as from all other recreational drugs for the duration of the trial * Willingness to abstain from prescribed psychostimulants for 72 h prior to Visits 2 and 3 (Day -1) and 24 h following IMP administration Further inclusion criteria apply. Exclusion Criteria: * Lifetime diagnosis of schizophrenia, schizoaffective disorder, schizophreniform disorder, bipolar I disorder, delusional disorder, autism spectrum disorder, or antisocial personality disorder as confirmed by the MINI * Any other psychiatric disorder that is not currently stable in symptoms and treatment * Any substance use disorder within 3 months prior to randomisation (excluding mild alcohol, cannabis, tobacco, and caffeine use disorders); or moderate to severe substance use disorder within the 6 months prior to randomisation (excluding tobacco and caffeine) * Positive drug screen. Participants with positive cannabis drug tests can be included if they do not meet criteria for moderate or severe cannabis use disorder and the investigator determines that use will not be an impediment to trial participation or accurate data collection * Concomitant use of psychotropic medication except for the ones below. All other psychotropic medications must be washed out at least 30 days or 5 Half-life time (t1/2) (whichever is longer) before the start of Visit 2 (Day -1) 1. A single SSRI (selective serotonin re-uptake inhibitor) or SNRI (selective serotonin and norepinephrine re-uptake inhibitor) antidepressant that has been stable in dose and frequency for \>3 months prior to randomisation 2. A single second-generation antipsychotic at a low dose that has been stable in dose and frequency for \>3 months prior to randomisation (low dose = 1 thorazine dose equivalent or less, which translates to ≤2 mg/day for risperidone, 5 mg/day for olanzapine, 75 mg/day for quetiapine, 60 mg/day for ziprasidone, and 7.5 mg/day for aripiprazole) 3. A single sleep medication given as a nightly scheduled medication (not pro re nata) stable in agent and dose for \>3 months prior to screening. Allowed sleep medications include: non-benzodiazepine Z sleep medications, antihistamines, melatonin, trazodone, and doxepin 4. Participants taking psychostimulant medication prescribed as per label for ADHD must stop medication 72 h prior to Visits 2 and 3 (Day -1) and may resume 24 h after receiving the medication dose on the test day (i.e. 5 days total off of prescribed psychostimulant for Visit 2 and 5 days off of prescribed psychostimulant for Visit 3) * 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 * A positive result for any active hepatitis * Previous randomisation in this trial Further exclusion criteria apply.