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Health Impacts of Cannabis

​​​​​​​​​​​​​​​​​​The evidence on the medical applications of cannabis is limited. There are established risks associated with regular, non-medical cannabis use. Ongoing research is improving what we know about both risks and benefits. For information on lower-ri​sk cannabis use, refer to Canada’s Lower-Risk Cannabis Use Guidelines.

Clearing the Smoke on Cannabis

The Clearing the Smoke on Cannabis series looks at how cannabis use affects mental and physical health, and discusses implications for policy and practice. Prepared by expert researchers in the field and peer-reviewed, this series addresses what we know about the health effects of cannabis use, what we don't know and what needs further investigation.

Cannabis regular use and cognitive functioning

Starting to smoke Cannabis earlier in life — before 16 or 17 — is one of the strongest predictors of noticeable cognitive difficulties. However, the impact of regular cannabis use on cognitive functions is generally mild for most people. Many of the measurable effects on these functions disappear after sustained periods of not using cannabis.

The report includes the following additional findings: 

  • Regular cannabis use is associated with changes in brain structure and function, including changes to the brain’s natural reward pathways.

  • Individuals who display risky and impulsive decision-making are more likely to develop problematic cannabis use and cannabis use disorder.

  • Establishing a standardized measurement of cannabis use, including frequency of use, dosage and methods of consumption, is required to explore the causal nature of these relationships. 

For more information, read Clearing the Smoke on Cannabis: Regular Use and Cognitive Functioning​.​

Chronic cannabis use and cognitive functioning and mental health​

Chronic cannabis use has been shown to impair various cognitive abilities, such as attention, memory and decision making. For most people, these impairments are relatively mild and are reversible after a few weeks of abstinence. However, current research suggests that chronic cannabis use:

  • Increases a person’s risk for developing psychotic symptoms and schizophrenia when started early in life, especially among people who might have a pre-existing genetic risk;

  • Increases the risk for psychosis for people who use cannabis that is high in delta-9-tetrahydrocannabinol (THC) and low in cannabidiol (CBD); and

  • Is associated with other mental health conditions, such as depression and anxiety; however, the nature of this relationship is not yet clear.

For more information, see Clearing the Smoke on Cannabis: Chronic Use and Cognitive Functioning and Mental Health.

Respiratory effects of smoking cannabis

Individuals who smoke cannabis on a regular basis commonly report coughing on most days, as well as wheezing, shortness of breath after exercise, chest tightness at night, sounds in their chest, early morning phlegm and bronchitis. Abstinence can reverse some of the negative respiratory symptoms experienced by those who smoke cannabis.
Research on the respiratory effects of smoking cannabis provides the following observations:

  • Cannabis smoke contains many of the same chemicals as tobacco smoke.

  • The link between cannabis smoking and lung cancer is mixed.

  • There is currently no causal evidence between cannabis smoking and COPD.

  • At present, it is unclear how vaping and second-hand cannabis smoke affect an individual’s respiratory system.

For more information, see Clearing the Smoke on Cannabis: Respiratory Effects of Cannabis Smoking.

Cannabis during pregnancy

An estimated 16.9% of individuals of childbearing age (15–44 years) reported past-year use of cannabis in 2015 and 2-5% of pregnant individuals reported using cannabis during pregnancy. There is little evidence to suggest an association between cannabis use during pregnancy and increased risk of premature birth, miscarriage or major physical abnormalities. However, frequent cannabis use during pregnancy is associated with:

  • Low birth weight and is part of a cluster of risk-factors correlated with other adverse birth outcomes;

  • Altered neurodevelopment and cognition, and academic under-achievement; and

  • Behavioural disturbances among children and young adults, including attention deficits, increased hyperactivity and impulsivity, and increased likelihood of delinquency and substance use.

For more information, see Clearing the Smoke on Cannabis: Maternal Cannabis Use during Pregnancy.

Cannabis and driving​

Cannabis impairs the cognitive and motor abilities necessary to safely operate a motor vehicle and doubles the risk of crash involvement. Current data supports the following conclusions:

  • Among young drivers in Canada, driving after using cannabis is more prevalent than driving after drinking.

  • Males are three times more likely than females to drive after using cannabis.

  • After alcohol, cannabis is the most commonly detected substance among drivers fatally injured in traffic crashes in Canada.

For more information, see Clearing the Smoke on Cannabis: Cannabis Use and Driving. For information on collisions caused by driving under the influence of cannabis, view the Cannabis Data page​.

Cannabis use during adolescence​

The Effects of Cannabis Use during Adolescence, a report in the Substance Use in Canada series, reviews the evidence on what we do and do not know about how cannabis affects adolescents The report addresses several important questions and key issues, including:

  • What are the brain and behavioural effects of cannabis use in youth?

  • Is there a link between cannabis and mental health?

  • Is cannabis addictive?

  • What interventions are available for cannabis use disorders?

The report concludes with A Call to Action. It gives parents, teachers, healthcare providers and policy makers the opportunity to develop and employ more effective youth drug use prevention and intervention programs.​

​For an online learning experience based on the report, see Online Learning for the Effects of Cannabis use during Adolescence.

Medical use of cannabis and cannabinoids

Promising research indicates that cannabis and cannabinoids are effective in relieving symptoms associated with certain severe and chronic conditions. As of June 2018, over 333,000 Canadians had registered to access cannabis for medical purposes.

  • Research is being conducted on using cannabis and cannabinoids to treat symptoms such as nausea, vomiting, loss of appetite and pain, and conditions such as multiple sclerosis, epilepsy, cancer, obesity and glaucoma, as well as inflammatory diseases and psychiatric and neurodegenerative disorders.

  • There are risks and harms associated with the medical use of cannabis, including lack of standardized dose information, and impacts on cognition and coordination.

  • ​Health practitioners and clients need more evidence, including the results of clinical trials, to support them in making informed decisions. 

Individuals should speak to their healthcare provider before making a decision about using cannabis for medical purposes.

For a detail discussion on this topic, see Clearing the Smoke on Cannabis: Medical Use of Cannabis and Cannabinoids.​