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Cannabis potency linked to higher risk of addiction and psychosis – 1st systematic review

Regular users of high-strength cannabis are up to five times more likely to develop addiction and psychosis, found a systematic analysis in The Lancet Psychiatry.

A University of Bath analysis found, with a survey of almost 120,000 people showing that those using products high in tetrahydrocannabinol (THC) run a greater risk of conditions like schizophrenia. The main psychoactive substance in cannabis, THC, affects how the brain works and influences a person’s mood, reactions, thoughts and emotions.

Data from the European Monitoring Centre for Drugs and Drug Addiction have revealed a 76% increase in people entering treatment for cannabis addiction in the past decade, the researchers said.

Those who use high-strength cannabis on a daily basis are five times more likely to develop a psyschotic disorder than those who never use it, according to one of 20 reviewed studies.

In contrast, found the researchers from the Addiction and Mental Health Group at the University of Bath, regular use of lower potency cannabis was not associated with an increased risk of psychosis. The analysis also found an increased risk of addiction, or “cannabis-use disorder”, among those who use high THC products compared with users of milder forms of the drug.

The study is thought to be the first systematic review of the available evidence on the link between cannabis strength and mental health problems and addiction. Cannabis is the third most widely used drug globally after alcohol and nicotine, with research suggesting around one in five 16 to 24-year-olds in the UK have used it in the past year.

The researchers say the findings could be vital in informing drug policy as more countries move to legalise cannabis, because THC concentrations in certain products have surged in recent years.

New legal markets have facilitated the development of high-THC cannabis extracts, as well as increasing the potency of herbal cannabis, the authors said.

Washington State in the US is one example where herbal cannabis of 20% THC concentration and cannabis extract of 60% THC concentration can be legally bought.

The study’s authors also noted that as potency has increased, so have rates of people seeking treatment for cannabis addiction.

Lead author Kat Petrilli, from the University of Bath’s Department of Psychology, said: “Our systematic review found that people who use higher potency cannabis could be at increased risks of addiction as well as psychosis compared with those who use cannabis products with lower potencies.

“These results are important in the context of harm reduction, which aims to minimise the negative consequences associated with drug use,” said Petrilli.

She added that while the safest level of use for cannabis “is, of course, ‘no use’, it’s important to acknowledge that a number of people worldwide use cannabis regularly and need to make informed decisions that could reduce any possible harms associated with it.”

Senior author Dr Tom Freeman added: “Our findings suggest that cannabis-users could reduce their risk of harm by using lower potency products.

“Where cannabis is legally sold, providing consumers with accurate information on product content and access to lower potency products could help safer use.”

The authors noted that despite anecdotal evidence of a link between cannabis use and anxiety and depression, there was no clear link between THC concentration and other mental health problems. Just one study included in the review suggested an association.

Their study was funded by the Economic and Social Research Council and published in The Lancet Psychiatry.

Study details

Association of cannabis potency with mental ill health and addiction: a systematic review

Kat Petrilli, Shelan Ofori, Lindsey Hines, Gemma Taylor, Sally Adams, Tom P Freeman.

Published in The Lancet Psychiatry on 25 July 2022

Cannabis potency, defined as the concentration of Δ9-tetrahydrocannabinol (THC), has increased internationally, which could increase the risk of adverse health outcomes for cannabis users. We present, to our knowledge, the first systematic review of the association of cannabis potency with mental health and addiction (PROSPERO, CRD42021226447). We searched Embase, PsycINFO, and MEDLINE (from database inception to Jan 14, 2021). Included studies were observational studies of human participants comparing the association of high-potency cannabis (products with a higher concentration of THC) and low-potency cannabis (products with a lower concentration of THC), as defined by the studies included, with depression, anxiety, psychosis, or cannabis use disorder (CUD). Of 4171 articles screened, 20 met the eligibility criteria: eight studies focused on psychosis, eight on anxiety, seven on depression, and six on CUD. Overall, use of higher potency cannabis, relative to lower potency cannabis, was associated with an increased risk of psychosis and CUD. Evidence varied for depression and anxiety. The association of cannabis potency with CUD and psychosis highlights its relevance in health-care settings, and for public health guidelines and policies on cannabis sales. Standardisation of exposure measures and longitudinal designs are needed to strengthen the evidence of this association.


The Lancet Psychiatry article – Association of cannabis potency with mental ill health and addiction: a systematic review (Open access)


The Independent article – High-strength cannabis use ‘increases risk of psychosis’


See more from MedicalBrief archives:


Trial shows cannabis extract helps reset brain function in psychosis


Cannabis and brain health – American Heart Association scientific statement


Cannabis abuse by US youth with mood disorders linked to self-harm, death



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