December 05, 2005

Cannabis-related Psychosis leads to Schizophrenia

A new scientific study published by Danish experts found that people who suffered a cannabis-induced psychosis were far more likely to go on to suffer from schizophrenia or other serious mental health problems.

The study is the first to show that temporary mental problems due to cannabis use are often followed by chronic psychiatric illness.

Researchers from Aarhus University Hospital found that nearly half of all patients treated for a cannabis-related psychotic episode went on to develop some form of schizophrenia.

And almost a third were later diagnosed as suffering from paranoid schizophrenia.

The research team tracked more than 530 patients treated for cannabis-induced psychosis for at least three years.

They were compared with more than 2,700 people with schizophrenia referred to psychiatrists, but who had no history of cannabis-induced mental health problems.

The team, writing in the British Journal of Psychiatry, said that their findings contradicted previous claims that cannabis-induced mental health problems were "harmless".

It found 44.5 per cent of those with cannabis-induced psychotic symptoms went on to suffer full-blown schizophrenia. Overall, more than 72 per cent went on to suffer from a psychotic illness such as depression.

Fewer than 16 per cent of the people made mentally ill by cannabis use required no psychiatric treatment after their initial psychotic episode.

The researchers also found that patients in the study group developed schizophrenia at an earlier average age than the non-drug using group - at 24, four years earlier, for men and at 29, three years earlier, for women.

Meanwhile, Professor Neil McKeganey the director of the Centre for Drug Misuse Research at Glasgow University, said that Scotland had one of the highest rates of cannabis use in Europe - and that users were starting at younger ages.

He said: "We should be looking at cannabis as a drug which will cause serious mental health problems for a proportion of the young people who use it.

"Even if it's a small proportion, the very large numbers of young people smoking cannabis over long periods means that a large number of young people may go on to suffer serious mental health problems.

"We are potentially facing a serious mental health issue for the large number of young people who are smoking cannabis in large amounts at a very young age, as low as 13 in some cases."

Of the Danish research, Prof McKeganey said: "It suggests we will see growing numbers of young people suffering from severe mental illness as a result of heavy drug use over a long number of years."

Source: Cannabis-induced psychosis and subsequent schizophrenia-spectrum disorders: follow-up study of 535 incident cases, British Journal of Psychiatry

For more information see: Marijuana, Cannabis and Schizophrenia


my son used cannabis prior to the time he had his first psychotic break. he was diagnosed with schizo-affective disorder. he began to hear voices and engage in an ongoing commentary when he was eight. we have a loaded family tree. my grandmother died in a mental hospital. my mothers brother was schizophrenic. my first cousin was schizophrenic. i was never told of these things but found out after my mother died and i had three children. my son 21 has schizoaffective disorder, my 35 year old daughter suffers from paranoid delusions, and my third child, a daughter, suffers from depression, as i do. i would not have had children had i known the facts, as i feel and see the suffering these illnesses bring.

Posted by: sv at January 12, 2006 06:46 PM

i suffered form marijuana induced psychosis, and i'm afraid that i have minor episodes spontaneously. i'm afraid to tell anyone, especially my parents.

Posted by: scared at May 5, 2006 11:53 AM

Please send me more information on this topic, I find this very interesting.
Thank you

Posted by: Jena at April 17, 2007 11:18 AM

The above-mentioned "new Danish study" looks very interesting. As I read the language I tried to find it on the net. Unfortunately, there doesn't seem to be even the faintest reference to it at all. Can anyone tell me where this information is supposed to come from? Has it been thoroughly checked? I fear my general trust in this whole website is at stake here.

Posted by: A Comment at June 15, 2007 01:59 PM

The link to the study's abstract was included. It was in the British Journal of Psychiatry. It is in English.

Posted by: Jeanie at June 16, 2007 07:52 AM

Thanks, Jeanie. I had overlooked that link. Looking at the report, now, I think it is fair to mention that it also concludes 'the incidence of cannabis-induced psychotic disorders in Denmark was estimated to be 2.7 per 100.000 person-years. This confirms that such conditions are rare.' The report furthermore lists several reasons why its results should be taken with more than just a small grain of salt. My personal opinion is that cannabis is a dangerous drug that causes grief in many people's lives. However, the group who develop clinical schizophrenia from it seems to be statistically very small (which of course doesn't make its suffering any smaller, and they have my deepest sympathy). An information campaign against the use of cannabis, which I think is the only way to deal with this problem, should probably be aimed at slightly different targets, such as the drug's tendency to induce milder forms of psychosis and schizophrenia-like conditions, which may not lead to psychiatric treatment but are still psychologically painful to the individual. Going back to the survey, however, its conclusions really are, in pure numbers of sufferers, considerably less dramatic than this web page presents them, which is probably why it was largely ignored by the Danish media at the time.

Posted by: A Comment at June 17, 2007 05:15 AM

More recent news is about what is happening in parts of Europe where the cannabis has been grown selecting for a greater percentage of THC which has resulted in less canabidiol. The number of hospital admissions has skyrocketed.
See here:

Apparently, the cannabidiol had some protective effect from the THC which increases the risk. See:

Some genes have been identified that conveys risk of developing schizophrenia from the THC. See here:

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