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Cannabis use significantly increases risk of ER visits and hospitalisation

Emergency room visits and hospitalisation are 22% higher among cannabis users, according to a Canadian study in BMJ Open Respiratory Research.

The large six-year retrospective cohort study, led by researchers at Unity Health Toronto and ICES, found serious physical injury and respiratory-reasons were the two leading causes of emergency department (ED) visits and hospitalisations among cannabis users.

The findings suggest an association between cannabis use and negative health events, which the researchers say should underline the need to educate and remind the public of the harmful impacts of cannabis on health. Recreational cannabis use has increased in Canada since decriminalisation in October 2018.

"Our research demonstrates that cannabis use in the general population is associated with heightened risk of clinically serious negative outcomes, specifically, needing to present to the ED or be admitted to hospital," said Dr Nicholas Vozoris, lead author, a respirologist at St Michael's and an associate scientist at the hospital's Li Ka Shing Knowledge Institute.

"Unlike tobacco, there is some uncertainty or controversy regarding the adverse health impacts of cannabis. Some individuals may perceive that cannabis has some health benefits and is otherwise benign. Our research highlights to those using – or considering to use – cannabis, that this behaviour is associated with important negative health events."

To compare health outcomes among cannabis users and individuals who don't use cannabis, researchers used data collected in a survey of individuals who self-reported cannabis use and linked it with health administrative data from ICES for Ontario residents ages 12 to 65 years old.

Using propensity score matching – a statistical matching technique – researchers compared the health outcomes of nearly 4,800 individuals who reported any cannabis use in the preceding 12 months with the health outcomes of over 10,000 individuals who reported never having used cannabis, or having used cannabis only once and more than 12 months ago. Researchers incorporated 31 different variables while matching study participants to minimize an unfair comparison, including demographics, multiple physical and mental health diseases, and tobacco, alcohol and illicit drug use.

The primary goal of the study was to explore whether there was a link between cannabis use and respiratory-related hospitalization or ED visits. The researchers found no significant associations between cannabis use and respiratory-related ED visits, hospitalizations, or death from any cause. However, they did find that overall visits to the ED or hospitalizations for any reason was significantly higher among cannabis users.

In addition to having greater odds of going to the ED or being hospitalized, the findings show that one of every 25 people who use cannabis will go to the emergency department (ED) or be admitted to hospital within a year of using cannabis.

Among the reasons cannabis users went to the ED or were hospitalised, acute trauma – defined as bodily injury – was the most common, with 15 per cent of cannabis users who got medical attention receiving it for this reason, and 14 per cent receiving care for respiratory reasons.

"The results of our research support that health care professionals and government should discourage recreational cannabis consumption in the general population. Given the context of cannabis decriminalization in Canada, which has very likely facilitated the broader use of this product in the population, more efforts need to made from our health and political leaders to educate and remind citizens about the harmful impacts of cannabis on health," said Dr Vozoris.

Study Details

Cannabis use and risks of respiratory and all-cause morbidity and mortality: a population-based, data-linkage, cohort study

Nicholas T Vozoris, Jingqin Zhu, Clodagh M Ryan, Chung-Wai Chow, and Teresa To.

Published in BMJ Open Respiratory Research on 28 June 2022



Although cannabis is frequently used worldwide, its impact on respiratory health is characterised by controversy.


To evaluate the association between cannabis use and respiratory-related emergency room (ER) visits and hospitalisations.


A retrospective, population-based, cohort study was carried out, linking health survey and health administrative data for residents of Ontario, Canada, aged 12–65 years, between January 2009 and December 2015. Individuals self-reporting cannabis use within the past year were matched to control individuals (people who reported never using cannabis, or used cannabis only once, and more than 12 months ago) in upwards of a 1:3 ratio on 31 different variables, using propensity score matching methods. Respiratory-related and all-cause ER visits or hospitalisations, and all-cause mortality, were evaluated up to 12 months following the index date.


We identified 35 114 individuals who had either used cannabis in the past year or were controls, of whom 6425 (18.3%) used cannabis in the past year. From this group, 4807 (74.8%) were propensity-score matched to 10 395 control individuals. In the propensity score matched cohort, there was no significant difference in odds of respiratory-related ER visit or hospitalisation between cannabis users and the control group (OR 0.91, 95% CI 0.77 to 1.09). Compared with control individuals, cannabis users had significantly increased odds of all-cause ER visit or hospitalisation (OR 1.22, 95% CI 1.13 to 1.31) and there was no significant difference with respect to all-cause mortality (OR 0.99, 95% CI 0.49 to 2.02).


Although no significant association was observed between cannabis use and respiratory-related ER visits or hospitalisations, the risk of an equally important morbidity outcome, all-cause ER visit or hospitalisation, was significantly greater among cannabis users than among control individuals. Therefore, cannabis use is associated with increased risk for serious adverse health events and its recreational consumption is not benign.


BMJ Open Respiratory Research article – Cannabis use and risks of respiratory and all-cause morbidity and mortality: a population-based, data-linkage, cohort study (Creative Commons Licence)


See more from MedicalBrief archives:


Cannabis smoking in teenage years linked to adulthood depression


Recreational cannabis legalisation significantly increases youth usage – large US study


Is marijuana as safe as we think?


Studies link marijuana to cardiovascular hazards



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