The results showed the expected association between tobacco smokers and reduced airflow. Those who smoked cigarettes alone, or cigarettes with cannabis, had reductions in their airflow over the 9 year period. Cannabis did not add to these reductions, over and above what was already found for tobacco only smokers. But perhaps surprisingly, in light of the recent findings on cannabis and emphysema, smoking cannabis alone did not reduce airflow or seem to impact lung functioning. Even after 9 years of use, cannabis smoke exposure did not seem to impact the lungs.
The authors concluded that “cannabis does not appear to be related to lung function, even after years of use.” They also concluded that using cannabis with tobacco doesn’t seem to add any additional risk to the lungs, beyond the harms already associated with smoking tobacco.
This is in stark contrast to the recent study in the journal Radiology suggesting smoking cannabis is more likely to cause emphysema than tobacco. In that study, chest CT scans revealed higher levels of emphysema in smokers who used cannabis and tobacco together, than those who used tobacco only. It’s important to note, however, that the Radiology study was limited by the fact that it did not look at any smokers who used cannabis alone. Thus the results suggesting higher rates of emphysema should be understood as relevant to the use of cannabis and tobacco together – not necessarily cannabis alone. There may be combinatory effects from mixing these two substances that aren’t present with either one alone. This doesn’t mean we can rule out cannabis as a potential cause of emphysema, but it does mean we need more research to confirm that these results hold for those using cannabis only.
In contrast, the Respiratory Medicine study did study cannabis-only smokers, and found no differences in lung functioning from the non-smoking control group.
The study in Radiology also used a relatively small sample size of only 146 patients, which can be compared to the 1173 respondents in the study that found no effects on lung function from smoking cannabis. That said, it’s important to note some other differences between these two studies that could partly explain their seemingly conflicting results.
First, the study in Radiology was on mostly older individuals, who have had more time to damage their lungs. It’s very possible that cannabis users would show more noticeable damage after more years of smoking than the 9 year period studied here. This study does not rule that out, it just shows no evidence of harm in the first 9 years.
It’s also important to note that the study in Radiology used CT scans to diagnose lung problems, while the recent study in Respiratory Medicine used spirometry. Different tests can produce different results. And in fact, some researchers suggest that spirometry tests, in particular, can miss conditions like emphysema in their early stages. So it may simply be that the damage being done isn’t being picked up by this test – the way it might be with the CT scan.
Given these research limitations, the question is still open on whether smoking cannabis can cause lung impairment and damage the way that tobacco more clearly does. We need more studies looking at the impacts of heavy cannabis smoking alone, throughout someone’s lifespan, before we can really come to a strong scientific conclusion.