Clinical Context

Research into the health effects of marijuana have traditionally been limited by its illegal status, but research has opened up gradually as legalization of marijuana has swept across the United States. One of the greatest areas of interest is the effect of chronic marijuana smoking on lung health. Tashkin previously performed a review of this subject, and the results were published in the September 2018 issue of Chest.^[1]

Most studies have linked marijuana smoking to a higher risk for chronic bronchitis. However, research has not linked marijuana to a decline in FEV₁ or diffusion capacity. In addition, a pooled analysis of 6 case-control studies plus a large cohort study failed to identify a relationship between marijuana smoking and the risk for lung cancer.

Two studies using high-resolution computed tomography (CT) imaging did not find an association between marijuana and the risk for emphysema. The current study revisits this issue and other potential effects of marijuana on pulmonary CT findings.

Study Synopsis and Perspective

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

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

Dr Revah said she can often tell immediately whether a CT scan is from a heavy or long-time cigarette smoker. But with the legalization 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.^[2,3]

She and her colleagues retrospectively examined chest CT scans from 56 marijuana smokers and compared them with scans of 57 nonsmokers and 33 users of tobacco alone.^[4]

Emphysema was significantly more common among marijuana smokers (75%) than among nonsmokers (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 may develop lung damage earlier or with less exposure, Dr Revah said.

Dr Revah 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 Alan Kaplan, MD, a family physician in Ontario who has expertise in respiratory health. He is the author of a 2021 review on cannabis and lung health.^[5]

In an editorial accompanying the journal article, pulmonary experts note that the new data give context to a recent uptick in referrals for nontraumatic pneumothorax.^[6] The authors say that they have received 22 of these referrals during the past 2 years, but that they had received only 6 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, Dr 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 is impossible to discern, Dr 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 program.

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

“The message to physicians is to ask about cannabis smoking,” Dr Kaplan said. In the past, people have been reluctant to admit using cannabis. Even with legalization, they may be slow to tell their physicians. But clinicians should still try to identify frequent users, especially those who are predisposed for lung conditions. If they intend to use the drug, the advice should be, “There are safer ways to use cannabis,” he said.

Dr Revah and Dr Kaplan have disclosed no relevant financial relationships.

Radiology. Published online November 15, 2022.

Clinical Implications

• A previous review found the closest association between marijuana smoking and the risk for chronic bronchitis. However, marijuana smoking was not associated with significant declines in FEV₁ or higher rates of lung cancer or emphysema.
• In the current study, marijuana smoking was linked to higher rates of emphysema, bronchial thickening, and mucoid impaction, although nearly 90% of marijuana smokers also had smoked tobacco products. Rates of coronary artery calcification were not increased in the marijuana group.
• Implications for the healthcare team: The healthcare team should consider the possible negative effects on pulmonary health when counseling patients who smoke marijuana.

Earn Credit