Psychosis occurs in a number of serious mental illnesses and not just schizophrenia - eg, depression, bipolar disorder (manic-depressive illness), puerperal psychosis and sometimes with drug and alcohol misuse. It can also occur in a number of neurological conditions and with drugs not associated with misuse.
This is a companion discussion topic for the original article at https://patient.info/doctor/mental-health/psychosis-diagnosis-and-management