Dr Philip Robson, Director of the Cannabinoid Research Institute, is an expert in the therapeutic potential of cannabis and cannabinoids.
Below are a list of definitions provided by Dr Robson relating to cannabis and mental health.
Psychosis: An organic or functional mental illness involving gross disorder of perception and/or thought form or content which causes the subject to lose touch with external reality.
Psychotic symptom: A symptom which causes a misinterpretation of the nature of reality. Typically hallucinations; delusional beliefs; disorders of the stream of thought (speed, pressure); formal thought disorder (linking of thoughts together).
Hallucination: A perception experienced in the absence of an external stimulus to the sense organs, with a similar quality to a true perception.
May be experienced by tired normal people, and not infrequently during the transition from sleeping to waking. It's only a psychotic symptom if the subject loses touch with external reference: e.g. a vision experienced during an LSD trip which the subject knows is due to the effect of the drug is not psychotic.
Hallucination has to be distinguished from an illusion (a misperception of an external stimulus).
Delusion: A belief that is firmly held despite compelling evidence to the contrary and is not a conventional belief that the person might be expected to hold given the particular educational and cultural background.
Paranoid: (or more accurately persecutory) ideas do not constitute a psychotic symptom unless held with delusional intensity (e.g. someone who feels spied upon, but is willing to concede that this is the result of smoking cannabis is not psychotic).
"Cannabis psychosis": It is generally accepted that cannabis, especially in high doses, can produce a transient, spontaneously remitting toxic psychosis in people with no history of severe mental illness.
The syndrome may include distorted time estimation, euphoria, fragmented thought processes, hallucinations, delusional beliefs, and altered levels of consciousness. There is no research evidence that I am aware of that links this reaction to any form of future mental illness.
Schizophrenia: This is the most serious form of functional (as opposed to organic) psychosis and consists of a variable range of disturbances of perception, thought, emotion, motivation, and motor activity.
Episodes of florid disturbance occur against a background of sustained disability. It is a disorder dogged by diagnostic difficulties.
The widely-used DSM-IV definition specifically excludes the diagnosis if key symptoms (bizarre delusions; third person auditory hallucinations; disorganized speech; grossly disorganized behaviour; negative symptoms) a) last less than 1 month and/or b) are "direct effect of drugs of abuse/medication or general medical condition".