Smoking marijuana and cigarettes may increase risk of lung problems compared to only smoking cigarettes, study suggests

• A study found higher rates of emphysema among marijuana smokers compared to tobacco smokers of the same age. 
• The study author told Insider her research raises concerns that marijuana smoking is not safer than cigarettes. 
• The results suggest smoking both marijuana and cigarettes is more harmful than smoking tobacco by itself.

A small study found higher rates of emphysema, a lung condition that causes shortness of breath, among marijuana smokers compared to tobacco smokers of the same age. The study also suggests marijuana and tobacco use together could be more harmful than tobacco use by itself. 

Dr. Giselle Revah, a cardiothoracic radiologist at The Ottawa Hospital and the study's lead author, looked at chest CT scans at Ottawa Hospital from 2005 to 2020 and identified 56 patients who reported using marijuana.

The majority of marijuana users — 50 out of the 56 patients — said they also smoked cigarettes. She compared them to 33 tobacco-only smokers and 57 non-smokers. 

The tobacco-only smokers' ages skewed higher because Revah collected these patient chest CTs through her hospital's lung cancer screening event, which was open to patients over 50 who self-reported as heavy smokers. Marijuana smokers in her sample tended to get a chest CT for reasons unrelated to emphysema. 

When the radiologist matched tobacco-only smokers to marijuana smokers of the same age, marijuana smokers had higher rates of emphysema: 93% (28 out of 30) compared to 67% of similarly-aged tobacco smokers. 

The radiologist found marijuana users overall — including younger people who hadn't been exposed to as much smoke — had significantly higher rates in particular of paraseptal emphysema, a rare form of the condition that damages tiny ducts which connect the lung's air sacs.

The way marijuana smokers use the drug might damage air sacs. Marijuana users tend to take deep breaths and hold smoke in for longer, causing pressure changes that can irritate the lung's air sacs, Revah told Insider.

"The main message of the whole study is there's this public perception that marijuana is safe; people believe that it's safer than cigarettes," Revah said. "And this study raises concerns that maybe marijuana's not as safe as everyone thinks it is, and suggests that ultimately we need more robust research before we can make sweeping conclusions."

The paper sheds light on the under-researched health effects of marijuana. Literature on the chest CTs of marijuana smokers is sparse, Revah said, since Canada only legalized the drug in 2018. The US has not legalized cannabis nationally, and getting funding for marijuana research involves cumbersome legal steps. 

Lung doctors told Insider more research is needed on the health effects of marijuana use. 

Dr. Philip Diaz, a pulmonary disease physician at the Ohio State University Wexner Medical Center who was not involved in the study, said because most marijuana smokers in the study were cigarette smokers, it's possible smoking both marijuana and cigarettes increases risk for lung damage. But Diaz stressed the results of the small study should not be overstated.

"You don't want to dilute the fact that it's really the cigarette smoking that's the problem," Diaz told Insider. "I think all you could say is there could be some increased risk if you do both."

Revah said she is working on a prospective study that asks patients how much marijuana they use, and hopes a larger study will confirm her findings. 

Dr. Albert Rizzo, the chief medical officer at the American Lung Association, told Insider scientists and doctors need longer, in-depth studies on the long term health effects of marijuana, especially as the drug quickly becomes legalized in states across the US. 

"I think this study's a good one in trying to show or support the fact that airway use of marijuana leads to problems, emphysema being among them," Rizzo told Insider. "Smoking marijuana is not safe, and we don't know really what the long term effect of smoking marijuana is."