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Essential Science: How does cannabis affect the brain?

Rates of cannabis consumption are increasing in many parts of the world, as countries (as with Canada) or the U.S. (as with the majority of states) either legalize consumption entirely or permit consumption for medical reasons.

There are different means of cannabis consumption, although inhalation (smoking and vaporizing) and ingestion are most common; and there are different grades of cannabis available. Smoking cannabis (or using a vaporizer) results in the psychoactive effect coming on faster but lasting for a shorter period of time, whereas ingesting cannabis leads to a delayed effect but with a longer-lasting ‘high’.

A cannabis plant.

A cannabis plant.
Michael Fischer (cc) via Pexels

Cannabis and mental illness

There have been reports and studies on the association (or not) between cannabis consumption and mental illness, dating back to the 1950s. This remains a widely researched subject and one for which there is greater imperative, given the increased rates of consumption and with cannabis now becoming a significant contributor to many economies through an array of businesses developing and marketing cannabis products.

The complexity with the debate leads to discussions about whether certain individuals already have mental health problems; whether there are underlying potentials for mental illness which are triggered by people entering a more relaxed state; and other data which suggests that cannabis can be used to treat certain forms of mental illness, such as depression.

As a sign of the complexity, two studies have recently been published which reach different conclusions relating to cannabis consumption and mental illness.

A one-ounce (28.3-gram) bag of cannabis and a bowl of seeds rest on a table at the headquarters of t...

A one-ounce (28.3-gram) bag of cannabis and a bowl of seeds rest on a table at the headquarters of the DC Cannabis Campaign on February 26, 2015, on the first full day of marijuana legalization in Washington
Robert MacPherson, AFP/File

No evidence of mental illness?

A review that compiles data from multiple research groups, gathered independently, which assessed the effects of cannabis exposure on the brain and intelligence scores, finds little impact of smoking cannabis on IQ scores or brain anatomy.

The studies include one from medical researchers at the University of California Los Angeles who used two longitudinal studies comparing adolescent twins. One of the twins smoked cannabis while the other did not. When the twins over time the researchers found no differences between the two in terms of IQ. This study is published in [i]PNAS[/i] and it is titled “Impact of adolescent marijuana use on intelligence: Results from two longitudinal twin studies.”

Another study cited considered brain scans on 781 young adults (aged 14-22), carried out by University of Pennsylvania scientists. The researchers found that, within the 147 cannabis users, there were no significant differences by cannabis group in global or regional brain volumes, cortical thickness, or gray matter density. This research has been published in the journal Neuropsychopharmacology and it is titled “Cannabis use in youth is associated with limited alterations in brain structure.”

A third piece of research found that long-term exposure to cannabis is not associated with changes in working memory. This Canadian clinical trial assessed the safety of daily cannabis consumption and here regular users demonstrated no identifiable differences in neurocognitive skills compared to non-using controls. The results here are published in the American Journal of Addiction, with the paper headed “Lack of hippocampal volume change in long-term heavy cannabis users.”

Although such studies indicate no association with mental illness, meta-analyses can only infer associations or lack of associations, rather than proving facts either way. Where such studies are useful is by promoting dialogue.

AFP file photo.

AFP file photo.
Jason Connolly, AFP/File

Potential mental illness issues?

A new study from Kings College London has been published in the journal The Lancet – Psychiatry. This study examines the association between regular cannabis use and psychosis.

The study looked a psychiatric patients from across ten European countries and Brazil, with the patients aged 18–64 years. The analysis showed that daily cannabis use was associated with an increased chance of developing a psychotic disorder compared with controls who had never used cannabis or cannabis products.

The factors having a stronger association with psychosis were identified as an increased frequency of cannabis use (that is the more often cannabis was used the greater the chance of developing a psychotic condition) and with the strength of the cannabis (that is, the stronger the potency of the cannabis (levels of tetrahydrocannabinol), then the greater the likelihood of developing psychosis).

The research also demonstrated how the association between cannabis use and risk of psychosis varies geographically depending on prevailing patterns of use. Pattern showed that the chances of having a psychotic disorder compared with controls were four times greater in Paris, five times greater in London, and more than nine times greater in Amsterdam, compared with the average proportion of cases across Europe and Brazil.

The research is titled “The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study.”

What does this all mean?

Is cannabis usage associated with an increased incidence or earlier onset of mental illness? Can any association said to be causal? How representative are the studies?

These questions cannot be answered straightforwardly and the answer remains ‘it depends’. There appears to be some association, but the reasons for that association may relate to individuals (rather than there being a collective impact), the frequency of use and the strength of the cannabis. It is a little cliché, but further research will be required.

Essential Science

Researchers installed the soft robotic gripper on a Fetch Robotics robot in their lab.

Researchers installed the soft robotic gripper on a Fetch Robotics robot in their lab.
University of California San Diego (Media)

This article is part of Digital Journal’s regular Essential Science columns. Each week Tim Sandle explores a topical and important scientific issue. Last week we looked at new technology from Massachusetts Institute of Technology, based on RFID tags, which can assist robots to gain greater precision for moving objects and also for increasing the speed of responses

The week before we investigated a study that finds that, overall, playing music whilst working inhibits the creative tide. This contrasts with other studies that show the opposite. Check out the story and see what your opinion is.

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Dr. Tim Sandle is Digital Journal's Editor-at-Large for science news. Tim specializes in science, technology, environmental, business, and health journalism. He is additionally a practising microbiologist; and an author. He is also interested in history, politics and current affairs.

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