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Marijuana Worsens Schizophrenia

Marijuana usage appears to worsen the symptoms of schizophrenia.

New research shows that when individuals diagnosed with schizophrenia use marijuana, they experience an increased ‘high,’ but symptoms of psychosis can worsen within a matter of hours.

Schizophrenia is a mental illness in which patients experience symptoms such as delusions, (a mistaken belief) hallucinations, sometimes a flattened affect, (where the individual shows little emotion) and disorganized behavior.  Up to 1% percent of the population is diagnosed with schizophrenia.  There is no known single cause, but genetics, as well as chemical and structural changes in the brain, may contribute to the development of the illness.  Treatment includes medications and therapy.

Dr. Cecile Henquet, an expert in both the study of marijuana use and psychosis, of Maasstrict University Medical Center in the Netherlands, and a team of researchers, compared a group of 42 schizophrenics who were daily marijuana users with 38 individuals without mental illness.  The study participants were asked what they were doing and how they felt twelve times a day for six days.

Immediately after using marijuana, the schizophrenic patients experienced an improved mood and feeling of well being that was significantly increased compared to the healthy study subjects.  However, several hours later, compared to the healthy individuals, the schizophrenic patients had more hallucinations, increased vulnerability to psychosis, decreased mood, and an overall worsening of disease symptoms.

It is suspected that the chemical compound responsible for the worsened symptoms is delta-9 tetrahydrocannabinol, commonly known as THC.

Prior research has shown that marijuana use by people at risk for mental illness can trigger hallucinations, delusions, and psychotic symptoms.  Schizophrenic users of marijuana don’t respond as well to medications, and have decreased memory function.

In 2005, Dr. Henquet’s team published a paper in the British Medical Journal, in which 2437 individuals between the ages of 14 and 24 were followed for 4 years.  The purpose of the study was to determine whether the individuals who had risk factors for psychosis were more likely to experience psychotic symptoms from marijuana use.  Although marijuana use may produce psychosis in healthy individuals, persons already at risk did in fact have more psychotic symptoms after drug use.

Additional research by Henquet showed that both genetic and environmental factors play a role in this predisposition.

Many researchers have postulated that schizophrenic patients have high rates of marijuana usage as a result of ‘self medication’ of symptoms such as depressed mood, disordered thinking, hallucinations and delusions.  Henquet’s latest findings confirm this theory by showing that patients with schizophrenia do experience an immediate improvement in mood that is more pronounced than in healthy individuals.  Thus marijuana use provides a strong immediate gratification.

Unfortunately, symptoms such as decreased mood appear within hours, prompting patients to use more marijuana, leading to a vicious cycle of abuse.

Henquet’s findings shed light upon this cycle of abuse and may give therapists insight into the reasons why schizophrenic patients have high rates of marijuana use.  New therapeutic interventions may include not only counseling patients to consider the negative consequences of drug use, but also acknowledging the positive effects.  Patients can then fully appreciate the costs of marijuana use.

Dr Henquet’s results are published in the June issue of the British Journal of Psychiatry.

Source: British Journal of Psychiatry

Marijuana Worsens Schizophrenia

Jessica Ward Jones, MD, MPH

APA Reference
Jones, J. (2018). Marijuana Worsens Schizophrenia. Psych Central. Retrieved on October 1, 2020, from
Scientifically Reviewed
Last updated: 8 Aug 2018 (Originally: 19 Jun 2010)
Last reviewed: By a member of our scientific advisory board on 8 Aug 2018
Published on Psych All rights reserved.