That is a large question.
Psychosis is a symptom and a loss of touch with reality. These could be fixed false-beliefs, which are delusions, or it could be hallucinations, which are false sensory perceptions, or you could have disorganized thought.
Whether someone has an intrinsic, organic psychiatric illness like schizophrenia or whether they have a substance-induced psychosis, such as cannabis-induced psychosis or amphetamine-induced psychosis, it's very difficult to tell these apart clinically.
One thing we have found is that people can have psychosis that is substance-induced that lasts a very long time. The textbook that we use in psychiatry, the Diagnostic and Statistical Manual, would describe a substance-induced psychosis as one month long. However, recent research from Shah et al found that 80% of cannabis-induced psychosis will last longer than a month. We know that with amphetamine-induced psychosis, 27% will last longer than a month. It's actually very common for these psychoses to last longer than a month, and clinically it's very difficult to tease these two apart.
It ends up being that the treatment is the same so you would use anti-psychotic medication to treat both.