Smoking cannabis was not associated with reduced lung function in young adults, according to a new longitudinal study published in Respiratory Medicine. Contrary to another recent study that suggested smoking cannabis is more likely to cause emphysema than smoking tobacco, researchers in this study found that smoking cannabis alone was not linked to reduced airflow or decreased lung function. Still, limitations in both studies suggest that we still don’t have a clear picture of the effect of smoked cannabis on our lungs.

This new study, conducted by researchers at the University of Queensland in Australia, examined whether chronic cannabis smoking was associated with harmful effects on the lungs of young adults in particular. “Cannabis use is increasingly legalized and use is normalized.” Lead author Professor Jake Najman explains. “In this context, it is important to understand more about the harms associated with long-term cannabis use.”

To investigate, researchers followed a cohort of 1173 young adults ages 21 to 30, testing their lung function using a spirometry assessment at the beginning and end of the 9-year period. Spirometry tests are commonly used to diagnose lung conditions, such as asthma and COPD, by measuring the amount of air a person can exhale in one forced breath. This is done with a device called a spirometer, which has a mouthpiece for the patient to breathe into.

Researchers also tracked whether cohort members smoked cannabis, tobacco, both, or nothing over the 9 years. This was done using questionnaires at ages 21 and 30, when lung function tests were performed. Then researchers analyzed the data to see if smoking cannabis over 9 years had reduced lung function.

The results showed the expected association between tobacco smokers and reduced airflow. Those who only smoked cigarettes, or cigarettes with cannabis, had a reduction in their airflow over the 9-year period. Cannabis did not contribute to these reductions, beyond what has already been found for smokers who only smoke tobacco. But perhaps surprisingly, in light of the recent findings on cannabis and emphysema, smoking cannabis alone did not reduce airflow and did not seem to affect lung function. Even after 9 years of use, exposure to cannabis smoke did not seem to affect the lungs.

The authors concluded that “cannabis appears to be unrelated to lung function, even after years of use.” They also concluded that using cannabis with tobacco does not seem to add any additional risk to the lungs beyond the damage already associated with tobacco smoking.

This is in stark contrast to the recent study in the journal Radiology, which suggests that smoking cannabis is more likely to cause emphysema than tobacco. In that study, CT scans of the chest revealed higher levels of emphysema in smokers who used cannabis and tobacco together than in those who used tobacco alone. However, it is important to note that the radiology study was limited by the fact that it did not look at cannabis-only smokers. Thus, the results suggesting an increase in emphysema should be understood as relevant to cannabis and tobacco use together – not necessarily cannabis alone. There may be combinatorial effects from mixing these two substances that are not present in either alone. This doesn’t mean we can rule out cannabis as a possible cause of emphysema, but it does mean that more research is needed to confirm that these results hold true for those who only use cannabis.

The respiratory medicine study, on the other hand, did study cannabis-only smokers and found no differences in lung function compared to the non-smoking control group.

The study in Radiology also used a relatively small sample of just 146 patients, which compares to the 1173 respondents in the study who found no effects on lung function from cannabis smoking. That said, it’s important to note some other differences between these two studies that could partially explain their apparently contradictory results.

First, the research in Radiology focused primarily on older individuals, who have had more time to damage their lungs. It is quite possible that cannabis users 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 damage 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, may miss early-stage conditions like emphysema. So it could simply be that the damage done is not picked up by this test – as it could be with the CT scan.

Given these research limitations, the question remains open as to whether smoking cannabis can cause lung damage and harm in the way that tobacco clearly does. We need more studies that look only at the effects of heavy cannabis smoking over a lifetime before we can really come to a strong scientific conclusion.

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