There are clinical reports of some individuals reaching psychosis-like states, like schizophrenia, depression or mania, after using marijuana. However, there is no marijuana-psychosis, more likely marijuana can trigger a person’s predisposed psychosis. That is, marijuana does not cause the psychosis, but an individual may already have a psychological disorder that is dormant and marijuana could possibly awaken the disorder. Marijuana alone does not produce a psychosis lasting past the period of intoxication.
People with psychiatric disorders are more likely to abuse drugs than the general population. Schizophrenics use marijuana in a higher percentage of the general population, but tend to not use alcohol or cocaine as much as the general population. Schizophrenics are at a higher risk for adverse psychiatric effects when using marijuana and heavy use can precipitate a schizophrenic episode. But marijuana does not cause the psychotic disorder.
There was a time that marijuana research made it appear as if there was a structural change in the brain of heavy marijuana users, but modern research has disproved those studies. While a person is intoxicated they tend to perform poorly in auditory functions, and this is due to reduction of blood flow to the temporal lobe of the brain.
Blood flow is actually increased to the frontal lobes and lateral cerebellum. Heavy marijuana users often can continue to have subtle defects in cognitive tasks up to one day of abstinence. There is little evidence that there are any longer term effects in cognitive ability, and little evidence that moderate users have any cognitive defects after the intoxication wears off.
While a person is under the influence of marijuana they have reduced psychomotor performance. This can include body sway, hand steadiness, rotary pursuit, driving and flying simulation, divided attention, sustained attention, and the digit-symbol substitution test. No one should drive a vehicle or operate dangerous equipment while under the influence of marijuana.
Amotivational Syndrome is a broad phrase often used to describe such things as the act of dropping out of social activities, or the lack of interest in school, work, or goal-related activities. While many people who have traits of the so called AS may use marijuana, marijuana is not the cause of their behavior. There is no causal relationship between marijuana smoking and AS.