Nav: Home

The Lancet Psychiatry: Daily use and high potency cannabis linked to higher rates of psychosis

March 19, 2019

  • Study is the first to show the impact of cannabis use on population rates of psychosis, highlighting the potential public health impact of changes to cannabis legislation
  • Link with psychosis was strongest in London and Amsterdam, where high potency cannabis is commonly available
  • In Amsterdam, an estimated 5 in 10 new cases of psychosis are linked with high potency use; in London, an estimated 3 in 10 new cases are linked with high potency use.
Daily cannabis use, especially of high potency cannabis, is strongly linked to the risk of developing psychosis, according to a case-control study from 11 sites across Europe, published in The Lancet Psychiatry journal. In cities where high potency cannabis is widely available, such as London and Amsterdam, a significant proportion of new cases of psychosis are associated with daily cannabis use and high potency cannabis.

Many countries have legalised or decriminalised cannabis use, leading to concerns that this might result in an increase in cannabis use and associated harms. Observational studies and biological evidence support a causal link between cannabis use and psychosis, but until now, it has been unclear whether, at a population level, patterns of cannabis use influence rates of psychosis.

Dr Marta Di Forti, lead author from the Institute of Psychiatry, Psychology, and Neuroscience at King's College London, UK, says: "Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms. They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level." [1]

"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties." [1]

The new study looked at 11 sites across Europe (and one in Brazil) [2]. First, the researchers estimated the prevalence of psychosis by identifying all individuals with first episode psychosis who presented to mental health services between 2010 and 2015. Secondly, they compared 901 patients with first episode of psychosis with 1,237 healthy matched controls to understand the risk factors associated with psychosis.

The researchers collected information about participants' history of cannabis use and other recreational drugs. Using published data on levels of delta-6-tetrahydrocannabinol (THC), they estimated cannabis potency for the types of cannabis used by participants, and classified types as either high potency (over 10% THC) or low potency (under 10% THC).

Daily cannabis use was more common among patients with first episode psychosis, compared to controls. 29.5% (266/901) of patients reported using cannabis daily, compared with 6.8% (84/1237) of controls. High potency cannabis use was also more common among patients with first episode psychosis, compared to controls. 37.1% (334/901) of patients reported using high potency cannabis, compared with 19.4% (240/1237) of controls.

Once adjusted for other factors, the authors found that across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first episode psychosis, compared with people who had never used cannabis. This increased to five times more likely for daily use of high potency cannabis.

The authors estimate that one in five new cases (20.4%) of psychosis across the 11 sites may be linked to daily cannabis use, and one in ten (12.2%) linked to use of high potency cannabis.

Use of high potency cannabis was a strong predictor of psychotic disorder in Amsterdam and London, where high potency cannabis is widely available. In the Netherlands, the THC content reaches up to 67% in Nederhasj, and 22% in Nederwiet; in London, skunk-like cannabis (average THC 14%) represents 94% of the street market, whereas in countries like Italy, France, and Spain, herbal types of cannabis with THC content of less than 10% were still commonly used.

For example, in Amsterdam, four in 10 (43.8%) new cases of psychosis were estimated to be linked to daily cannabis use, and 5 in 10 (50.3%) new cases linked to high potency use [3]. Corresponding rates in London were 21.0% for daily use, and 30.3% for high potency use.

If high potency cannabis were no longer available, the incidence of psychosis in Amsterdam would be expected to drop from 37.9 to 18.8 per 100,000 people per year; and in London from 45.7 to 31.9 per 100,000 people per year.

The authors note that the data on cannabis use was not validated by biological measures such as urine, blood or hair samples. But they note that the cut-off of 10% THC for potency is conservative. The estimates on potency also do not include the proportion of cannabidiol (CBD), another important component of cannabis.

Writing in a linked Comment, Dr Suzanne H Gage, University of Liverpool (UK), asks whether we can now be sure that daily and high potency cannabis use causes psychosis. "Unfortunately, not all the evidence utilising different methods is consistent about causality... It is perfectly possible that the association between cannabis and psychosis is bidirectional. [This study] adds a new and novel study design to the evidence available, which consistently indicates that for some individuals there is an increased risk of psychosis resulting from daily use of high potency cannabis. Given the changing legal status of cannabis across the world, and the associated potential for an increase in use, the next priority is to identify which individuals are at risk from daily potent cannabis use, and to develop educational strategies and interventions to mitigate this."
-end-
Peer-reviewed / Case-control study / People

NOTES TO EDITORS

The study was funded by the Medical Research Council, the European Community's Seventh Framework Program grant, Sao Paolo Research Foundation, National Institute for Health Research (NIHR) Biomedical Research Centre (BRC) at South London and Maudsley NHS Foundation Trust and King's College London and the NIHR BRC at University College London, and the Wellcome Trust.

The labels have been added to this press release as part of a project run by the Academy of Medical Sciences seeking to improve the communication of evidence. For more information, please see: http://www.sciencemediacentre.org/wp-content/uploads/2018/01/AMS-press-release-labelling-system-GUIDANCE.pdf if you have any questions or feedback, please contact The Lancet press office pressoffice@lancet.com

[1] Quotes direct from authors and cannot be found in the text of the Article.

[2] Sites included in the study: London (UK), Cambridge (UK), Amsterdam (Netherlands), Gouda and Voorhout (Netherlands), Paris (Val-de-Marne, France), Puy de Dôme (France), Madrid (Spain), Barcelona (Spain), Bologna (Italy), Palermo (Italy), Ribeirão Preto (Brazil).

[3] Data derive from the Population Attributable Fraction (PAF) which measures the population effect of an exposure by providing an estimate of the proportion of cases that would be prevented if the exposure (ie, daily/high potency cannabis use) were removed. The PAF assumes causality between daily/high potency cannabis use and first episode psychosis.

IF YOU WISH TO PROVIDE A LINK FOR YOUR READERS, PLEASE USE THE FOLLOWING, WHICH WILL GO LIVE AT THE TIME THE EMBARGO LIFTS: http://www.thelancet.com/journals/lanpsy/article/PIIS2215-0366(19)30048-3/fulltext

The Lancet

Related Mental Health Articles:

Food insecurity can affect your mental health
Food insecurity (FI) affects nearly 795 million people worldwide. Although a complex phenomenon encompassing food availability, affordability, utilization, and even the social norms that define acceptable ways to acquire food, FI can affect people's health beyond its impact on nutrition.
Climate change's toll on mental health
When people think about climate change, they probably think first about its effects on the environment, and possibly on their physical health.
Quantifying nature's mental health benefits
The BioScience Talks podcast features discussions of topical issues related to the biological sciences.
Sexism may be harmful to men's mental health
Men who see themselves as playboys or as having power over women are more likely to have psychological problems than men who conform less to traditionally masculine norms, according to research published by the American Psychological Association.
Mental health matters
UCSB researchers study the effectiveness of an innovative program designed to help youth learn about mental health.
Could mental math boost emotional health?
Engaging the brain's dorsolateral prefrontal cortex (DL-PFC) while doing mental math may be connected with better emotional health, according to Duke researchers.
Program will train mental health providers, improve health care in rural Missouri
A new graduate education program at the University of Missouri has received nearly $700,000 from the Health Resources and Services Administration in the US Department of Health and Human Services to train psychology doctoral candidates in integrated, primary health care settings, in an effort to improve health care for underserved populations with mental health and physical disorders.
Loss of employer-based health insurance in early retirement affects mental, physical health
The loss of private health insurance from an employer can lead to poorer mental and physical health as older adults transition to early retirement, according to a study by Georgia State University.
Ocean views linked to better mental health
Here's another reason to start saving for that beach house: new research suggests that residents with a view of the water are less stressed.
New study shows electronic health records often capture incomplete mental health data
This study compares information available in a typical electronic health record (EHR) with data from insurance claims, focusing on diagnoses, visits, and hospital care for depression and bipolar disorder.

Related Mental Health Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#SB2 2019 Science Birthday Minisode: Mary Golda Ross
Our second annual Science Birthday is here, and this year we celebrate the wonderful Mary Golda Ross, born 9 August 1908. She died in 2008 at age 99, but left a lasting mark on the science of rocketry and space exploration as an early woman in engineering, and one of the first Native Americans in engineering. Join Rachelle and Bethany for this very special birthday minisode celebrating Mary and her achievements. Thanks to our Patreons who make this show possible! Read more about Mary G. Ross: Interview with Mary Ross on Lash Publications International, by Laurel Sheppard Meet Mary Golda...