What effects does smoking cannabis have on the respiratory system?

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The most common form of administration of cannabis (either recreational or medicinal) is through inhalation (smoking or vaporizing).¹²  A 2016 survey of 364 Canadian medicinal cannabis users found that 52.7% vaporized cannabis, whereas 58.8% also smoked cannabis in some form.²   As cannabis contains many compounds with a similar profile to tobacco, heating and combustion of these chemicals may result in short- and long-term respiratory effects.³  This Evidence Brief will summarize what is known about smoking cannabis and its acute and chronic effects on the respiratory system.

Similarities and differences between smoking cannabis and tobacco

When comparing cannabis and tobacco smoke, the exposures are different based on chemical constituents and smoking topography.  Cannabis smokers typically inhale more deeply and for a longer duration.⁵ Cannabis is also smoked at higher temperatures (leading to greater combustion) and, in cigarette (“joint”) form, smoked to a shorter butt length.⁵ Furthermore, it has been demonstrated that the net respiratory burden of particulates is four times greater with cannabis than tobacco smoking. ⁵ As cannabis possesses many of the same harmful and carcinogenic chemicals as tobacco smoke (Table 1), it is theorized that acute and chronic smoking may lead to similar harm with respect to the respiratory system such as bronchitis, COPD and lung cancer.

Acute Effects on Lung Function

Upon initial use, inhaling cannabis (in particular, via THC) may dilate the airways of healthy and asthmatic individuals from several minutes to hours and might be dose related. ⁶⁷ Anandamide, an endocannabinoid, is known to bind to CB₁ receptors in the lung, serving to relax bronchial muscles.⁶  However, when bronchial muscles are already relaxed, anandamide will cause the opposite effect and produce bronchospasms, therefore restricting airways. ⁶   It is thought that chronic administration of THC, a phytocannabinoid, may also result in relaxing bronchial muscles acutely, but result in bronchoconstriction with chronic administration.³⁶

Chronic Effects on Lung Function

With chronic (habitual) smoking of cannabis, users often cite symptoms such as coughing, wheezing, shortness of breath, excess sputum production, chest tightness, pharyngitis, hoarse voice, worsening asthma symptoms and the development of chronic bronchitis.³  The odds of developing such symptoms as a result of chronic cannabis use (controlling for tobacco use), are 2-3 times greater when compared to individuals who have never smoked cannabis. ³ One interesting finding is that when comparing tobacco and cannabis smokers, the chronic respiratory symptoms of a cannabis smoker were similar to a tobacco smoker who had smoked for an additional 10 years. ³

Chronic smoking results in physical changes to the respiratory system.  When looking at biopsy specimens, researchers have noted that chronic cannabis smoking results in extensive histopathologic abnormalities in the trachea-bronchial mucosae similar to tobacco use.³  These findings have also been confirmed upon visual inspection with video bronchoscopy, where smoked cannabis use results in large airway inflammation and injury. ³

Infection Risk

In addition to airway injury, cannabis smoke may increase the risk of airway infections such as pneumonia.  Cannabinoids are known to modulate, and possibly blunt immune cells such as B/T lymphocytes, natural killer cells, suppressing phagocytosis by macrophages and altering the presence of cytokines ⁶. Airway infection rates among HIV-positive cannabis smokers are increased compared to non-users. ⁶    In addition, as the cannabis plant can harbor fungal and bacterial pathogens, there have been reports of allergic bronchopulmonary aspergillosis due to Aspergillus fumigatus in cannabis-users with asthma and cystic fibrosis. ⁶  Individuals sharing devices, such as pipes and bongs, have also been found to be at increased risk of tuberculosis (~2 to 6 fold risk).⁷

Chronic Obstructive Pulmonary Disease (COPD) Risk

When reviewing epidemiological studies, occasional use of marijuana with low cumulative use has not been established as a risk factor for the development of COPD (based on measuring spirometry).³  However, heavy use may increase risk of COPD, particularly amongst individuals who smoke both cannabis and tobacco. ³  It should be noted that the available evidence to inform this issue is limited by the small number of cannabis users studied, and the variability in use from 50 joints to greater than 10,000 joints per lifetime. ³  Hence the association between chronic cannabis smoking and the development COPD remains to be established.

Lung Cancer Risk

As cannabis smoke contains carcinogens, there is a concern that cannabis smoking may lead to the development of lung cancer. With chronic cannabis use, histological changes have been noted in biopsies pointing to pre-cancerous states.⁶ There have been approximately 12 studies to date reporting on the risk of cannabis smoking and lung cancer.⁸  These studies were conducted in the US, Europe and North Africa and most involved both men and women. ⁸ Eight studies noted an increase in lung cancer risk from cannabis use and four studies found either no significant association or a lower risk for lung cancer. ⁸In those studies that noted an increased risk, these ranged from 2 to 4-fold increased risk of lung cancer in cannabis smokers, but were not methodologically as strong as those studies finding no increases in cancer risk.⁶⁸  In conclusion, cannabis use is plausibly relevant to lung cancer risk but the evidence has been inconsistent.

Reversal of Adverse Effects via Cannabis Smoking Cessation

It is unknown whether cannabis smoking cessation reduces the observed histological changes in airways.³ However, there are benefits to cannabis smoking cessation.  A longitudinal study of 299 young adults over 10 years found that those who smoked either cannabis or tobacco were at increased risk of symptoms of chronic bronchitis at follow-up. ⁹  In comparison, those individuals who quit smoking either cannabis or tobacco had a similar risk of chronic bronchitis symptoms as those who never smoked. ⁹  These findings were also noted in a cohort study of 1000 subjects over 38 years, that found stopping cannabis, tobacco or cannabis/tobacco smoking significantly reduced the amount of coughing, sputum secretion and wheeze, and that reported rates of these events that were similar to those who never smoked. ¹⁰

Conclusion and Key Takeaways

• Inhaling cannabis, particularly smoking, remains one of the most common methods of administration of medicinal cannabis
• Cannabis possesses many of the same harmful chemicals as tobacco smoke, either in similar or greater quantities (Table 1)
• Cannabis can lead to similar risk of harm to the respiratory system (Table 2)
• Methods other than smoking or vaporizing should be emphasized, such as oral administration of cannabis, due to potential risks of chronic bronchitis, infection, COPD and lung cancer

Glossary of Terms

Aspergillosis – is the name given to a wide variety of diseases caused by infection by fungi of the genus Aspergillus.

Asthma – a chronic lung disorder that is marked by recurring episodes of airway obstruction.

B/T lymphocytes – a form of small leukocyte (white blood cell) with a single round nucleus, occurring especially in the lymphatic system.

Bronchial, Bronchi – is a passage of airway in the respiratory system that conducts air into the lungs after branching from the trachea.

Bronchitis – is an inflammation of the lining of the bronchial tubes.

Bronchopulmonary – Pertaining to both the air passages (bronchi) leading to the lungs and the lungs (pulmonary) themselves.

Bronchoscopy – Bronchoscopy is an endoscopic technique of visualizing the inside of the airways for diagnostic and therapeutic purposes.

Bronchospasm – spasm of bronchial smooth muscle producing narrowing of the bronchi.

Confounding – A situation in which the effect or association between an exposure and outcome is distorted by the presence of another variable.

Cytokines – any of a number of substances, such as interferon, interleukin, and growth factors that are secreted by certain cells of the immune system and have an effect on other cells.

Endocannabinoid – cannabinoids produced endogenously (for example in humans) and not from plant sources.

Epidemiological – relating to the branch of medicine which deals with the incidence, distribution, and control of diseases.

Histopathologic – refers to the microscopic examination of tissue in order to study the manifestations of disease.

Macrophages – a large phagocytic cell found in stationary form in the tissues or as a mobile white blood cell, especially at sites of infection.

Mucosae – is a membrane that lines various cavities in the body and covers the surface of internal organs.

Natural killer cells – are a type of lymphocyte (a white blood cell) and a component of innate immune system.

Particulates – matter in particulate form that can cause harm to human health.

Pathogens – a bacterium, virus, or other microorganism that can cause disease.

Phagocytosis – the ingestion of bacteria or other material by phagocytes.

Pharyngitis – is caused by swelling in the back of the throat (pharynx) between the tonsils and the voice box (larynx).

Phytocannabinoid – naturally occurring cannabinoids found in the cannabis plant.

Pre-cancerous – (of a cell or medical condition) likely to develop into cancer if untreated.

Spirometry – measures lung function, specifically the amount (volume) and/or speed (flow) of air that can be inhaled and exhaled.

Sputum – a mixture of saliva and mucus coughed up from the respiratory tract.

Trachea – colloquially called the windpipe, is a cartilaginous tube that connects the pharynx and larynx to the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs.

Tuberculosis – is a lung disease caused by bacteria called Mycobacterium Tuberculosis.