Only a small percentage of the estimated 300 000 people who smoke cannabis in Holland become psychotic. As far as we know it only concerns people who are consciously or unconsciously sensitive for psychosis. However, it is possible that cannabis can turn out badly with "healthy" people as well and cause anxiety or depression. Fortunately, these symptoms will not last and quickly
disappear. There is no proof whatsoever that long-term and daily use of cannabis can cause a psychosis among "healthy" people, but the risk cannot be totally ignored. There are examples of people in India who - after years of daily use - started to show symptoms similar to psychosis, like hallucinations, delusions and total introversion. However, these are just descriptions of cases
and are not scientifically proven.
So cannabis is almost positively harmful to people who tend to psychosis. Therefore, in 1993 research was done at the Academic Medical Center in Amsterdam (AMC) among 93 psychotic patients. It
showed that 61% of the patients who used cannabis during 15 months, more than once a day, fell back into a psychosis. Almost every user suffering from psychosis turned out to use cannabis at least a year before their first psychosis. According to the researcher it meant that the use of cannabis by vulnerable people could result in developing a psychosis. (Source: Don Linszen et al., Archives of General Psychiatry 1994).
Further research to the use of cannabis by people with a tendency to psychosis shows that:
Relapse in a psychosis occurs more often among cannabis users than non-cannabis users.
Cannabis has a negative effect on the course of the psychosis.
Users of cannabis suffering from a psychosis become psychotic faster, more heavily and more sustained.
Users of cannabis with a tendency to psychosis become psychotic at a younger age than non-users.
Medication necessary for psychotic patients is less effective when someone uses cannabis. At the same time the side effects are less harmful. Often that is one of the reasons why people use cannabis.
Psychotic people often use cannabis because it decreases the symptoms of the psychosis. It is used as a form of self-medication against fears, especially social fears like difficulty in talking in public. Patients indicate that it was easier and they felt better when they had used cannabis.
Note that using cannabis is illegal in most countries.