In the Bergen Psychosis Project 2 the antipsychotic drugs aripiprazole, amisulpride, and olanzapine will be compared head-to-head in patients with schizophrenia and related psychoses and followed for 12 months. The study is independent of the pharmaceutical industry, and in accordance with a pragmatic design a clinically relevant sample will be included with as few exclusion criteria as possible. The patients will be assessed repeatedly with regards to symptoms, side effects, and cognitive functioning, as well as laboratory parameters. The study hypothesis is that clinically meaningful differences among the drugs will be disclosed in a pragmatic design.

Inclusion Criteria:

A:The observational cohort

• Patients 16 years old or older
• Active psychosis as determined by a score of 4 or more on one or more of the items Delusions, Hallucinatory behavior, Grandiosity, Suspiciousness/ persecution, or Unusual thought content in the Positive and Negative Syndrome Scale (PANSS).
• Or ICD-10 diagnosis corresponding to psychotic disorders or other mental disorders with psychotic features (F10-F19: .5 (psychotic disorder); F20-F29, F30.2, F31.2, F31.5, F32.3, F33.3). From which eligible patients are recruited to the B:The pragmatic, randomized, controlled trial (The Best Intro Study)
• Patients 18 years and older
• Schizophrenia spectrum and delusional disorder
• Symptoms of psychosis as determined by a score of 4 or more on one or more of the items Delusions, Hallucinatory behavior, Grandiosity, Suspiciousness/ persecution, or Unusual thought content in the Positive and Negative Syndrome Scale (PANSS).

Exclusion Criteria:

• Inability to understand spoken Norwegian.
• Patients with organic psychosis due to limbic encephalitis detected by antibodies in serum obtains at inclusion (such as NMDAR, VGKC and paraneoplastic antibodies performed at the Neuroimmunology Laboratory, Department of Neurology, Haukeland University Hospital) Pregnant or breast feeding women.
• Aripiprazole: Hypersensitivity to the active substance or to any of the excipients
• Amisulpride: Hypersensitivity to the active ingredient or to other ingredients of the medicinal product; concomitant prolactin-dependent tumours e.g. pituitary gland prolactinomas and breast cancer; phaeochromocytoma; lactation, combination with the following medications which could induce torsade de pointes: Class Ia antiarrhythmic agents such as quinidine, disopyramide, procainamide. Class III antiarrhythmic agents such as amiodarone, sotalol. Other medications such as bepridil, cisapride, sultopride, thioridazine, methadone, IV erythromycin, IV vincamine, halofantrine, pentamidine, sparfloxacin. Combinations with levodopa.
• Olanzapine: Hypersensitivity to the active substance or to any of the excipients. Patients with known risk of narrow-angle glaucoma.