Heavy cannabis users - who are prone to destructive lung disease sometimes known as "bong lung" - can suffer irreversible lung damage, a study suggests.
Smoking cannabis can cause bronchitis and, in heavy users, can lead to irreversible lung damage, the new study from the University of Otago shows.
Respiratory specialists from Otago and Waikato Hospital reviewed research from New Zealand and overseas involving thousands of people and found heavy users were prone to a destructive lung disease sometimes known as 'bong lung'.
Author and University of Otago Professor Bob Hancox said heavy users could end up with badly damaged lungs for life.
"The bronchitis that people get, the really nasty bronchitis, does tend to improve if you stop.
"But what we as lung doctors sometimes see in people that don't stop smoking cannabis - we see people coming in with lungs which are very very badly damaged with lots of destruction of the lung tissue, and that is irreversible."
Professor Hancox said much of the cannabis debate centred on the social and mental health effects, but the impact on lungs also needed attention.
"I do think the debate should consider the lung health effects and I haven't heard much discussion of these effects at all."
He said the pattern of lung damage caused by cannabis was different - not necessarily worse - but different to smoking tobacco.
For example, smoking tobacco could cause a spectrum of lung disorders known as chronic obstructive pulmonary disease, or COPD.
"There is convincing evidence for symptoms of chronic bronchitis, but the pattern of lung function changes is different to tobacco and there is insufficient evidence to conclude that cannabis causes COPD," the paper concluded.
Professor Hancox said there was still a lot that was not known about cannabis, but it was clear that the drug was not harmless to the lungs.
"Whether liberalising availability will lead to further increases in cannabis use remains to be seen, but it is likely that patterns of cannabis use will change, with resulting health consequences."
Cannabis is the second-most commonly smoked substance after tobacco and the most widely used illicit drug worldwide.
Challenges with studying cannabis
Cannabis has been difficult to study as it remains illegal and unregulated in most countries, another of the study's authors, Dr Kathryn Gracie, said.
Dr Gracie, from Waikato Hospital's Respiratory Department, said the fact most cannabis users also smoked tobacco also made the direct effects on the lungs difficult to separate.
"Perhaps, most importantly, the individuals who are extremely heavy users of cannabis may not be well represented in the existing epidemiological research."
Heavy users could be reluctant to come forward for large studies due to fear of prosecution.
There were some case reports of cannabis-related destructive lung disease in very heavy cannabis consumption, Dr Gracie said.
"Despite these limitations there is sufficient evidence that cannabis causes respiratory symptoms and has the potential to damage both the airways and the lungs.
"Cannabis may also increase the risk of lung cancer, but there is not enough evidence to be sure of this yet," Dr Gracie said.
A combination of smoking both cannabis and tobacco is likely to mean people "are likely to get the worst of both substances," Professor Hancox said.
The authors did not look at studies related to vaping cannabis, which made headlines last year when more than 2000 people in the United States suffered lung injuries that appeared to be linked to vaping and cannabis.
"It's not yet clear if it's the cannabis in the vape or other things causing the lung damage, so we haven't addressed that in this review," Prof Hancox said.
There was a "big question" about Covid-19 and the effects of vaping, Professor Hancox said.
"There's almost no evidence yet because it's all so new, but it seems likely that if you're vaping and bringing out clouds of vape and you happen to have Covid, that doesn't seem like a very safe thing to do."
The study was published in the journal Addiction.