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Emergency department visits and hospitalizations for cannabis use during pregnancy almost doubled since legalization, study finds


In one of the largest studies of changes in cannabis use in pregnancy after legalization, researchers from ICES, The Ottawa Hospital, and Unity Health Toronto found that the number of pregnancies that required treatment for cannabis use in the emergency department (ED) or during a hospitalization in Ontario increased by 82% (from 11 to 20 per 100,000 pregnancies) following legalization.

The researchers also found that babies of people who visited the ED or were hospitalized for cannabis use during pregnancy had very high rates of severe adverse outcomes, such as low birth weight, preterm birth, and admission to the neonatal intensive care unit (NICU).

“While these events—capturing very harmful patterns of cannabis use—are fortunately rare, we are concerned that they may reflect much larger increases in overall cannabis use in pregnant people following legalization,” says lead author and ICES post-doctoral trainee Daniel Myran, who is also a family physician and fellow at the Bruyère Research Institute and The Ottawa Hospital.

Published in CMAJ, the study included data for 980,398 pregnancies from 691,242 individuals between January 2015 and June 2021 in Ontario, Canada. Of the study population, 533 pregnant people had one or more pregnancies with an acute care visit (ED visit or hospitalization) for cannabis, with the majority of the events (72%) being ED visits. The most common reasons included harmful cannabis use (58%), cannabis dependence or withdrawal (22%) and cannabis poisoning (13%).

The researchers also found a significant link between treatment for severe morning sickness and treatment for cannabis use during pregnancy and noted that this relationship has grown even stronger since legalization.

“Severe morning sickness—or what is called 'hyperemesis gravidarum'—was frequently seen among individuals who received an acute care visit for cannabis use in pregnancy,” says senior author Andrea Simpson, adjunct scientist at ICES and an obstetrician and minimally invasive gynaecologic surgeon at St. Michael’s Hospital, a site of Unity Health Toronto. “Although we can’t say for sure whether they were presenting with vomiting as a result of cannabis use, or using cannabis to treat morning sickness, it does raise concern that more pregnant individuals perceive cannabis use in pregnancy to be less risky since legalization.”

The data showed that:

  • Legalization was associated with a gradual increase over time in acute care visits for cannabis during pregnancy, which is consistent with increases in ED visits for cannabis use among the general population.
  • Over the same time period there was no increase in acute care for other types of substance use (e.g. alcohol or opioids) during pregnancy and a decline in acute care for mental health conditions like depression and anxiety during pregnancy
  • Compared to the general pregnant population, individuals visiting the ED or being hospitalized for cannabis use were younger (24 vs 30 years) and more likely to live in the lowest-income neighbourhoods (40% vs 22%) and rural settings (17% vs 9%).
  • 32% of babies of pregnancies that received care for cannabis use were admitted to the NICU, 12% were in the smallest 5th percentile of birth weight, and 17% were born pre-term before 37 weeks gestation. After adjusting for key differences between the groups, pregnancies with acute care for cannabis continued to be associated twice the odds of these severe outcomes compared to pregnancies without.

This is an observational study, so cause and effect can’t be established. The researchers highlight that there could be other factors that are leading to the high risk of severe adverse outcomes in newborns in the study. Nevertheless, they say their findings add to the evidence of harm from cannabis use during pregnancy, and that people with heavy cannabis use could benefit from high-risk obstetrical care.

“There is a strong need for further interventions aimed at reducing cannabis use during pregnancy, including improved awareness of potential harms and screening for cannabis use” says Dr. Myran. “Our findings suggest that several higher risk groups, including individuals with severe morning sickness, may benefit from additional non-stigmatizing screening and counselling”.

The study, “Acute care due to cannabis during pregnancy after the legalization of non-medical cannabis in Ontario: a population-based, repeated cross-sectional study”, was published in CMAJ.

Author block: Myran DT, Roberts R, Pugliese M, Corsi D, Walker M, El-Chaâr D, Tanuseputro P, Simpson A.

ICES is an independent, non-profit research institute that uses population-based health information to produce knowledge on a broad range of healthcare issues. Our unbiased evidence provides measures of health system performance, a clearer understanding of the shifting healthcare needs of Ontarians, and a stimulus for discussion of practical solutions to optimize scarce resources. ICES knowledge is highly regarded in Canada and abroad, and is widely used by government, hospitals, planners, and practitioners to make decisions about care delivery and to develop policy. In October 2018, the institute formerly known as the Institute for Clinical Evaluative Sciences formally adopted the initialism ICES as its official name. For the latest ICES news, follow us on Twitter: @ICESOntario

The Ottawa Hospital
(TOH) is one of Canada’s top learning and research hospitals where we are guided by our vision to provide the world-class and compassionate care we would all want for our loved ones. Our multi-campus hospital, affiliated with the University of Ottawa, is home to the Regional Trauma Centre and Cancer Centre, and to discoveries that are adopted globally. Backed by generous support from the community, we are focused on reshaping the future of healthcare to improve the health of our diverse population of patients from Eastern Ontario, Western Quebec, and Nunavut. For more information about The Ottawa Hospital, visit OttawaHospital.on.ca

The University of Ottawa
is home to over 50,000 students, faculty and staff, who live, work and study in both French and English. Our campus is a crossroads of cultures and ideas, where bold minds come together to inspire game-changing ideas. We are one of Canada’s top 10 research universities—our professors and researchers explore new approaches to today’s challenges. One of a handful of Canadian universities ranked among the top 200 in the world, we attract exceptional thinkers and welcome diverse perspectives from across the globe. www.uottawa.ca

About St. Michael's

St. Michael’s Hospital provides compassionate care to all who enter its doors. The hospital also provides outstanding medical education to future healthcare professionals in more than 27 academic disciplines. Critical care and trauma, heart disease, neurosurgery, diabetes, cancer care, care of the homeless and global health are among the Hospital’s recognized areas of expertise. Through the Keenan Research Centre and the Li Ka Shing International Healthcare Education Centre, which make up the Li Ka Shing Knowledge Institute, research and education at St. Michael’s Hospital are recognized and make an impact around the world. Founded in 1892, the hospital is fully affiliated with the University of Toronto.

About Unity Health Toronto

Unity Health Toronto, comprised of St. Joseph’s Health Centre, St. Michael’s Hospital and Providence Healthcare, works to advance the health of everyone in our urban communities and beyond. Our health network serves patients, residents and clients across the full spectrum of care, spanning primary care, secondary community care, tertiary and quaternary care services to post-acute through rehabilitation, palliative care and long-term care, while investing in world-class research and education. For more information, visit www.unityhealth.to.


Misty Pratt
Senior Communications Associate, ICES
[email protected] 613-882-7065

Amelia Buchanan
Senior Communication Specialist
Ottawa Hospital Research Institute
Cell: 613-297-8315
[email protected]

Marlene Leung
Senior Communications Advisor, Unity Health Toronto
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