One Method Drugs Might Induce Psychosis
I think psychosis is widely considered to represent a kind of split in the personality/memory/experience, to cope with a trauma or existential discontinuity.
Here is one effect of psychedelics: surfacing material from the subconscious into conscious awareness. Surfacing a difficult constellation of emotions, memories, or confusions could itself be difficult.
Here is one point about cannabis and psychosis: a person who experiences an existential trauma (for them), such as a debilitation, could possibly have psychotic reaction to cannabis because of the characteristic manner in which cannabis can surface the trauma. [For example, I would suggest the word, "disassociation" if a person in weed-induced psychosis started talking about "distractibility."]
I would be less concerned about the risk of LSD or mushrooms surfacing trauma in a manner that induces psychosis (personally). The manner in which these substances surface traumatic issues seem to 'go with the subject' more, step-by-step. It is as if the drugs say, "Think about it all the way through...Feel it...Yes...Let out that ocean of tears, baby..." or "Here is a summary of your entire life, flashing before your eyes and summed up neatly in one hour, just to give you another look..."
I suppose psychedelics could be traumatic. For example, being given a psychedelic against one's will, when it is not right for that person. It could possibly cause deep trauma (as well as surface trauma) in such a manner as to induce psychosis.
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The legal-research that is being done on psychedelics has excluded people with pre-existing / family mental health problems. This is not definitive one way or the other. Obviously, early research should be done in such a manner to demonstrate the positive healing potential of these drugs. These drugs can help people to heal in various ways--so the strategy is to prove their efficacy and safety, get them legalized for prescription purposes--and afterwards explore more uncertain problems from a contemporary medical methodology such as relieving (or possibly exacerbating) mental illness.
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As far as I understand about psychosis, there is something called 'reality testing'--is the person capable of testing their 'delusions'? Like, if a client believes that a doctor is reacting to the client's thoughts--is the client capable of putting this theory to a 'reality test'? The majority view seems to focus on reducing the effects of psychosis, as if it were a painful disease that gets worse and worse, rather than letting psychosis 'run its course,' as if it were a healing process. Terrence McKenna once described his brother's week-long(er) psychosis when they were living off of the magic mushroom fields they discovered during their deep wanderings in the jungle. Terrence insisted that the only reason his brother healed, is that they did not try to 'slap him out of it,' but that they let the psychosis proceed along its natural course until it came to a point of resolution.