Respiratory health effects of cannabis—How should we respond to liberalization of cannabis laws?

Cannabis (marijuana) is the world's most commonly-used illicit drug.1 While it remains illegal in most countries, several countries and states have recently legalized it and policies around recreational or medicinal cannabis are progressively being liberalized in many other places. In New Zealand, where I live, a referendum to legalize recreational cannabis was narrowly lost in 2021, but the trend towards greater tolerance and an eventual change in its legal status seems inevitable. Much of this is to be welcomed—including cannabis in the 'war on drugs' has undoubtedly led to many social harms including criminalisation of young people for what is perceived to be a normative behaviour, providing a lucrative business for criminal gangs, and denying some people access to medicinal cannabis treatments that they believe to be highly effective. However, what has been missing in this process of liberalization is robust evidence on the potential harms and benefits of cannabis (whether used for recreational or medicinal purposes).2 While there has been considerable discussion about the mental and cognitive effects of cannabis among young people, the respiratory health effects have received little attention. Overall, the physical harms of cannabis appear to be less than those of tobacco,2, 3 but being safer than one of the world's most lethal substances is not a reassuring comparison. One of the reasons that we have so many uncertainties about the long-term effects of cannabis on the lungs is that its illegal status has made it hard to study. People are understandably reluctant to report that they have been using an illegal drug, there is no regulation of supply or quality, and the concentration of the psychoactive ingredient, Δ9-tetrahydrocannabinol, is variable making dosing difficult to interpret. It is also challenging to distinguish the effects of cannabis from those of tobacco because many users smoke both. Nevertheless, we have sufficient evidence to conclude that smoking cannabis has adverse effects on respiratory health and that these effects are different to those of tobacco.4, 5 So what do we know? At the severe end of the spectrum of cannabis-induced lung disease there are numerous case-reports of severe upper-lobe bullous emphysema, sometimes associated with pneumothorax (Figure 1).4 This clinical presentation has been called 'bong lung' (although there is no evidence that this is more likely in people who inhale cannabis from a bong rather than other methods of smoking it). However, we do not yet have epidemiological evidence that smoking cannabis causes emphysema: no association has been found in case–control or cohort studies using high-resolution CT scans although one study found a weak association with impaired gas transfer, consistent with early emphysema.5-7 The apparent absence of emphysema among most cannabis users suggests that bong lung only occurs in susceptible individuals after very extensive cannabis use. At the other end of the spectrum, cannabis use is strongly associated with cough, sputum production, and, to a lesser extent, wheeze and dyspnoea. These symptoms often occur in relatively light cannabis users (once a week or more) and improve on reducing or stopping cannabis.8, 9 Bronchoscopy studies show bronchial inflammation at much less frequent levels of cannabis use compared to tobacco smokers. Why cannabis is such a potent cause of bronchitis is unclear. Perhaps cannabis smoke is exceptionally irritative, suppresses immune responses to infection, or the method of smoking it (at high temperatures without filters) is particularly damaging to airway mucosa.10 Regardless of the mechanism, this has clinical implications. Patients with cough should be asked about cannabis and can be advised that their symptoms are likely to improve if they stop smoking it. Curiously, leading international cough guidelines do not refer to cannabis and it is not mentioned as a potential 'treatable trait' in the recent British Thoracic Society statement on chronic cough.11-13 Despite the strong association with bronchitis, there is little evidence that smoking cannabis leads to COPD. Several studies have found lower Forced Expiratory Volume in 1 second to Forced Vital Capacity (FEV1/FVC) ratios among cannabis smokers, but, unlike tobacco, this is mostly due to higher FVCs rather than a decline in FEV1.4, 5 This increase in FVC is accompanied by higher Total Lung Capacity (TLC) and hyperinflation on CT scans.5-7 Why this occurs is not clear. Cannabis is associated with lower specific airway conductance, which may lead users to breathe at higher lung volumes to overcome this.4, 5 Alternatively it has been suggested that the typical method of smoking cannabis (deep inhalation to TLC followed by a prolonged breath-hold) may lead to hyperinflation. Regardless of the mechanism, it is clear that smoking cannabis can change lung physiology leading to lower FEV1/FVC ratios. This is not COPD as we usually understand it, but perhaps an adult-acquired form of dysanapsis: a mismatch between airway calibre and lung size. Dysanapsis is not necessarily benign: it has been linked to higher all-cause mortality.14 We don't yet know for sure is whether cannabis is a risk factor for lung cancer.15 Although some studies have found a link, most have not. Cannabis contains many of the toxic ingredients found in tobacco10 and it remains highly plausible that it is carcinogenic, but the epidemiological evidence is not yet sufficient. Where next for the respiratory community? We can't afford to ignore the potential impact of cannabis on the lungs. Global cannabis use is increasing and liberalization is likely to lead to greater use among at least some of the population.1, 16 Cannabis use will remain substantial even in countries that continue to prohibit its use. We have enough evidence that smoking cannabis harms respiratory health and, even though the extent of this harm is not yet clear, the precautionary principle suggests that we should raise these concerns with our patients. We should also raise our concerns in debates about the legal status of cannabis—although legalization and decriminalization have strong arguments to commend them, we should remind people of the potential respiratory effects and ask what steps will be taken to mitigate these. Finally, we should advocate for more and better research. We shouldn't wait for cannabis to be legalized before we investigate its risks and benefits. Open access publishing facilitated by University of Otago, as part of the Wiley - University of Otago agreement via the Council of Australian University Librarians. None declared.