No, cannabis is no more dangerous than tobacco

Last week, a Canadian study was widely reported in the media as «cannabis is more harmful to the lungs than tobacco».

But the study doesn't say that. Here's what the study actually found.

No cannabis-only smokers were included

Rule number one for recently published studies: Read the results for yourself. Unless the source of the information is clearly indicated, be skeptical about the conclusions drawn from new research until you review it yourself.

In this case, a quick glance at the study in question - « Chest CT Findings in Marijuana Smokers« published on November 15 in Radiology - reveals a major problem.

The chest X-ray results were not from cannabis smokers. They were from tobacco smokers who also smoked cannabis. No cannabis-only smokers were included in the study.

The study also concludes that the way it was set up «limits our ability to draw firm conclusions».

How the study was carried out

The study's lead author, a diagnostic radiologist at the Ottawa Hospital in Ontario, Canada, wrote: «The aim of this study was to use chest CT to investigate the effects of cannabis use on the lungs. We sought to determine if there were any identifiable sequelae on the chest CT images, including emphysema and signs of airway inflammation.»

To create test groups, researchers examined the medical records of patients seen between 2005 and 2020. The study states, «Thirty-three smoking-only patients were identified with chest CT scans performed between April and June 2019.»

«If cannabis use was identified, the patient was excluded» from the tobacco smoking group, «added to the cannabis smoking group, and a new patient was selected.»

In other words, the study did not compare tobacco smokers to cannabis smokers. Instead, the researchers compared tobacco smokers only to tobacco smokers who also smoked cannabis. There were no groups who smoked cannabis but not tobacco.

Probably not a good idea to smoke cigarettes and cannabis

The study did not reveal that «cannabis can do more harm to smokers than cigarettes alone». But that's the conclusion that was picked up by several sites.

In fact, the study authors discovered that smoking both tobacco and cannabis is not healthy. «The study authors found bronchial thickening in 64 % of cannabis smokers versus 42 % of tobacco-only smokers, as well as a condition leading to excessive mucus accumulation in 23 % of cannabis smokers versus 6 % of tobacco-only smokers.».

It's not clear whether this applies more to those who smoke cigarettes and cannabis separately, or to those who mix tobacco with their cannabis.

What are the findings of previous studies?

A 2007 study established a link between emphysema and other lung problems in cannabis smokers and non-smokers, but tobacco or cigarettes were not part of the research.

Another study carried out in 2012 by the University of Alabama at Birmingham (UAB) concluded that «marijuana smoke (is) not as harmful to the lungs as cigarette smoke», with a much larger sample size and more rigorous methodology.

«The data showed that even up to moderately high levels of use - one joint a day for seven years - there was no evidence of decreased airflow or lung volumes.»

The UAB study found: «At the exposure to cannabis commonly observed in Americans, occasional cannabis use was associated with increased lung airflow and lung capacity,» said the study's lead researcher. Nevertheless, «these increases were not large, but they were statistically significant,» according to the published study.

This study recognizes that lifetime exposure to smoke can still be harmful, but also that long-term heavy cannabis users are not easy to find for studies, given the plant's uncertain legal status.

The UAB study was funded by the National Heart, Lung and Blood Institute. It was a long-term research project involving over 5,000 black and white men and women from Birmingham, Chicago, Minneapolis and Oakland, designed to examine the development and determinants of cardiovascular disease and its risk factors. Participants were recruited when they were between 18 and 30 years of age and were followed from 1985 to 2006.