A long-term study tracking just over a thousand participants has found regular cannabis smoking can change the function of your lungs as you age. Unlike smoking a cigarette, however, cannabis seems to impact a person's breathing in a slightly different way.
Over adulthood, tobacco smoking is associated with a progressive decline in how much air you can force from your lungs in a given amount of time. In comparison, cannabis smoking in the current study was connected to higher lung volumes in total.
Ultimately, the authors found both changes lead to similar end results – constricting airways and leading to hyperinflation and gas trapping.
These patterns of lung function match previous results from the same cohort, which were collected 13 years earlier when the participants were 32 years of age.
"Although the effects of cannabis were detrimental, the pattern of lung function changes was not the same," says respiratory specialist Bob Hancox from the University of Otago in New Zealand.
"The research found that prolonged cannabis use led to over-inflated lungs and increased the resistance to airflow to a greater extent than tobacco."
To this day, there is limited research on the lung effects of smoking cannabis. Some studies suggest smoking cannabis flower can lead to acute bronchitis-like symptoms, while other studies suggest even after seven years of smoking, lung function is not significantly altered.
One of the main challenges is teasing apart the effects of cannabis from tobacco, as most cannabis users are also tobacco smokers.
The participants included in the current study from New Zealand were no exception. Most were smokers of both tobacco and cannabis, but even among those who had never smoked tobacco before, the authors found similar patterns in lung function.
While it's easy to measure how many cigarettes a day a person smokes, however, there is currently no standardized form of a joint. As such, the current study could only differentiate between daily cannabis smokers and those who smoke less than once a week.
This probably underestimates how much cannabis some participants are consuming, which makes it hard to say how much you need to smoke to put the health of your lungs at risk.
The heaviest cannabis users in the study tended to be tobacco smokers as well, confounding the results even more.
"Cannabis users tend to smoke far fewer times a day than tobacco smokers and it is possible that the participants have not smoked enough cannabis for it to have a measurable effect on some aspects of lung function," the authors note.
"However, this seems unlikely in view of the strong associations with higher lung volumes and lower airway conductance."
These associations were especially strong and consistent among those who smoke both tobacco and cannabis. In fact, these participants showed a slight tendency towards lower gas transfer, meaning their lungs weren't as good at exchanging oxygen for carbon dioxide.
The authors worry this is putting some individuals at risk of emphysema, although it's still not clear what these changes in lung function do to a person's overall lung health.
"[I]t is increasingly clear that cannabis has different effects on lung function to tobacco and the effects of widespread cannabis use will not necessarily mirror the harms caused by tobacco smoking," the authors conclude.
"The long-term consequences of the hyperinflation, gas-trapping, and lower airway conductance observed in several studies are not yet known."
Cannabis users who smoke regularly have been reporting changes to their lungs for years now, including a susceptibility to bronchitis. But this is one of the first studies to put the drug to the test in longitudinal research.
Cannabis has many upsides, but it also has some downsides. The way that it harms the lungs is largely unstudied, and given the increasing popularity of this drug, both for medicine and recreational purposes, it's worth finding out more.
The participants in the current study have had measurements taken of their lung capacity since they were 18 years of age, and their lungs all seemed to start at roughly the same baseline of health.
By age 45, however, the toll of smoking was clear to see, especially amongst those who smoked tobacco.
Further research will need to explore whether vaping could cut down on some of the lung effects associated with long-term cannabis use, but even those who quit or cut down on their cannabis smoking in the current study did not show a substantial improvement in their lung function.
"In other words, their lung function after quitting was not better than the function at the last measurement when they were still smoking," Hancox told ScienceAlert in an email.
"It is possible that a longer period of time without cannabis would lead to improvements, but we didn't see this yet."
Other studies by Hancox and colleagues have shown cutting down on cannabis does seem to improve bronchitis-like symptoms, including coughing and wheezing. Whether lung functions can be similarly reversed remains to be seen.
The study was published in the American Journal of Respiratory and Critical Care Medicine.