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Higher lung damage risk for marijuana smokers than tobacco smokers – Canadian study

Scans of the lungs of marijuana users have turned up an alarming surprise: regular smokers appear to be at greater risk of lung damage, including emphysema, than are people who smoke tobacco alone.

“There’s a public perception that marijuana is safe,” said Dr Giselle Revah, a radiologist at the University of Ottawa, Ontario, Canada. “But this study is raising concern that this might not be true.”

Revah said she could often tell immediately if a CT scan was from a heavy or long-time cigarette smoker. But with the legalisation and increased use of marijuana in Canada and many US states, she began to wonder what cannabis use does to the lungs and whether she would be able to differentiate its effects from those of cigarette smoking.

Medscape reports that she and her colleagues retrospectively examined chest CT scans from 56 marijuana smokers and compared them to scans of 57 non-smokers and 33 users of tobacco alone.

Emphysema was significantly more common among marijuana smokers (75%) than among non-smokers (5%). When matched for age and sex, 93% of marijuana smokers had emphysema, vs 67% of those who smoked tobacco only (P = .009).

Without age matching, rates of emphysema remained slightly higher among the marijuana users (75% vs 67%), although the difference was no longer statistically significant. Yet more than 40% of the marijuana group was younger than 50 years, and all of the tobacco-only users were 50 or older, meaning that marijuana smokers might develop lung damage earlier or with less exposure, Revah said.

She added that her colleagues in family medicine have said the findings match their clinical experience. “In their practices, they have younger patients with emphysema,” she said.

Marijuana smokers also showed higher rates of airway inflammation, including bronchial thickening, bronchiectasis and mucoid impaction, with and without sex- and age-matching, the researchers found.

The findings are “not even a little bit surprising”, according to Dr Alan Kaplan, a family physician in Ontario, who has expertise in respiratory health. He is the author of a 2021 review on cannabis and lung health.

In an editorial accompanying the article in the journal Radiology, pulmonary experts note that the new data give context to a recent uptick in referrals for non-traumatic pneumothorax. The authors say they have received 22 of these referrals during the past two years but that they had received only six between 2012 and 2020.

“Many, but not all, of these patients have a documented history of marijuana use,” they write.

One reason for the additional damage may be the way marijuana is inhaled, Kaplan said. Marijuana smokers “take a big breath in, and they really push it into lungs and hold pressure on it, which may actually cause alveoli to distend over time”.

Because most marijuana smokers in the study also smoked cigarettes, whether the observed damage was caused by marijuana alone or occurred through a synergy with tobacco was impossible to discern, Revah said.

Still, the results are striking, she said, because the marijuana group was compared with tobacco users who had an extensive smoking history – 25 to 100 pack-years – and who were from a high-risk lung cancer-screening programme.

Revah and her colleagues are now conducting a larger, prospective study to see whether they can confirm their findings.

Study details

Chest CT Findings in Marijuana Smokers

Luke Murtha, Paul Sathiadoss, Jean-Paul Salameh, Matthew Mcinnes, Giselle Revah.

Published in Radiology on 15 November 2022


Global consumption of marijuana is increasing, but there is a paucity of evidence concerning associated lung imaging findings.

To use chest CT to investigate the effects of marijuana smoking in the lung.

Materials and Methods
This retrospective case-control study evaluated results of chest CT examinations (from October 2005 to July 2020) in marijuana smokers, nonsmoker control patients, and tobacco-only smokers. We compared rates of emphysema, airway changes, gynecomastia, and coronary artery calcification. Age- and sex-matched subgroups were created for comparison with tobacco-only smokers older than 50 years. Results were analysed using χ2 tests.

A total of 56 marijuana smokers (34 male; mean age, 49 years ± 14 [SD]), 57 nonsmoker control patients (32 male; mean age, 49 years ± 14), and 33 tobacco-only smokers (18 male; mean age, 60 years ± 6) were evaluated. Higher rates of emphysema were seen among marijuana smokers (42 of 56 [75%]) than nonsmokers (three of 57 [5%]) (P < .001) but not tobacco-only smokers (22 of 33 [67%]) (P = .40). Rates of bronchial thickening, bronchiectasis, and mucoid impaction were higher among marijuana smokers compared with the other groups (P < .001 to P = .04). Gynecomastia was more common in marijuana smokers (13 of 34 [38%]) than in control patients (five of 32 [16%]) (P = .039) and tobacco-only smokers (two of 18 [11%]) (P = .040). In age-matched subgroup analysis of 30 marijuana smokers (23 male), 29 nonsmoker control patients (17 male), and 33 tobacco-only smokers (18 male), rates of bronchial thickening, bronchiectasis, and mucoid impaction were again higher in the marijuana smokers than in the tobacco-only smokers (P < .001 to P = .006). Emphysema rates were higher in age-matched marijuana smokers (28 of 30 [93%]) than in tobacco-only smokers (22 of 33 [67%]) (P = .009). There was no difference in rate of coronary artery calcification between age-matched marijuana smokers (21 of 30 [70%]) and tobacco-only smokers (28 of 33 [85%]) (P = .16).

Airway inflammation and emphysema were more common in marijuana smokers than in nonsmokers and tobacco-only smokers, although variable interobserver agreement and concomitant cigarette smoking among the marijuana-smoking cohort limits our ability to draw strong conclusions.


Radiology article – Chest CT Findings in Marijuana Smokers (Open access)


Marijuana and the vulnerable lung: the role of imaging and the need to move quickly (Open access)


Medscape article – Lung Damage Worse in Pot Smokers (Open access)


See more from MedicalBrief archives:


More American adults now smoke cannabis than cigarettes


Cannabis legalisation a ‘significant concern’ – American Heart Association


Is marijuana as safe as we think?


Americans say marijuana, vaping less harmful than tobacco – Gallup




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