Marijuana use and schizophrenia: New evidence suggests link

by Yvette Brazier

A new study, published in Psychological Medicine, has added to the body of evidence pointing to a link between schizophrenia and the use of cannabis.

Recent research suggests that not only are people who are prone to schizophrenia more likely to try cannabis, but that cannabis may also increase the risk of developing symptoms.

Studies show that cannabis use is more common among people with psychosis than in the general population, and that it may also increase the risk of psychotic symptoms.

Its use has been linked to symptoms of psychosis, such as paranoia and delusional thinking, in up to 40 percent of users.

Earlier this year, scientists warned that young people who use cannabis could be putting themselves at risk of psychotic disorders. People with schizophrenia appear to have a higher chance of experiencing psychosis if they use cannabis.

Previous warnings had voiced concerns regarding the particularly powerful strains of cannabis, such as "skunk," currently circulating among young people.

However, the findings were not considered definite and experts called for more research.

Genome data reveals possible link

Now, Dr. Suzi Gage and team, from the School of Experimental Psychology at Bristol University in the United Kingdom, have discovered more evidence that cannabis may be particularly hazardous to people with schizophrenia, although they highlight that other factors also impact mental health.

Fast facts about cannabis

• Around 44 percent of 12th graders have used marijuana in their lifetime
• An estimated 46 percent of people in the United States aged 26 and over have used it
• Approximately 12.8 percent of 8th graders have used the drug, with 0.7 percent using it daily.

Learn more about marijuana

The authors looked at genetic factors that may predict whether a person is likely to use cannabis, and whether they are likely to develop schizophrenia. The data was taken from a wide-ranging genome study.

The Mendelian Randomization technique, used to analyze the data, enabled the authors to account for a range of variants.

This technique was chosen because the team predicted that people who use cannabis are likely to be genetically and biologically different from those who do not in a variety of ways.

Findings indicate that starting to use cannabis may increase the risk of schizophrenia, but especially that a person who is at risk of schizophrenia is more likely to use cannabis.
This may be because genetic factors for schizophrenia are stronger than those for cannabis use.
The study authors would like to investigate whether a genetic link might explain the extent of cannabis use, because results show that people with schizophrenia are more likely to be heavy users of cannabis.

Medical News Today asked the authors why people with a risk of schizophrenia might be more likely to use the drug.

Prof. Marcus Munafò, a member of the team, speculated that "certain behaviors or symptoms associated with schizophrenia risk might be relieved by the effects of cannabis." In other words, cannabis use may be a kind of self-medication in this population.

An alternative explanation, he said, is that "people with higher risk of schizophrenia may enjoy the psychological effects of cannabis more."

"There is growing consensus that cannabis use might increase the risk of developing schizophrenia. Our results support this, but also suggest that those at increased risk of schizophrenia may be more likely to try cannabis in the first place."

Prof. Marcu Munafò

MNT asked the researchers what role cannabis might play in raising or lowering the risk of schizophrenia.

Prof. Munafò pointed out that while further studies are needed, existing research suggests that two of the constituents of cannabis, tetrahydrocannabinol (THC) and cannabidiol (CBD), might have an impact.

The roles of CBD and THC

THC intoxication has been associated with transient psychotic experiences. CBD, on the other hand, does not trigger mind-altering experiences and may have potential as a medication, according to the National Institute on Drug Abuse (NIDA).

NIDA point out that most recreational marijuana is high in THC and low in CBD.

Dr. Gage and colleagues predict that the proportion of THC to CBD will probably be significant in psychotic symptoms.

In addition, Prof. Munafò told MNT that results will be needed from multiple sources, including laboratory and epidemiological studies, before any definite conclusions can be reached.

Limitations of the study include the relatively small overlap between cannabis users and people with schizophrenia in around 1,500 people.

Until now, there have been too few consistent studies to compare the effects of cannabis, and as a result, assessing the impact of cannabis on brain structures has remained a challenge. However, it has been found that people with schizophrenia who use cannabis are more likely to be hospitalized than those with the condition who do not use the drug.

Dr. Gage expresses an interest in looking more closely at the subgroups of more vulnerable cannabis users, in order to gain a better understanding of how heavy usage might influence their well-being.