Respiratory specialists are calling for cannabis legislators to consider the impacts of smoking the substance after a review found links to bronchitis and a severe disease known as "bong lung".
New Zealand researchers looked at available international research on the effects of smoking cannabis on the lungs, and found while it behaved differently to tobacco, even smoking small amounts could lead to respiratory issues.
University of Otago's Professor Bob Hancox said while much of the debate about legalising cannabis revolved around the social and mental health effects, policies around the liberalisation should consider the wider health effects of smoking.
"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," Hancox said.
Cannabis is the second-most commonly smoked substance after tobacco and the most widely-used illicit drug world-wide.
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Although cannabis remains illegal in most countries, many countries – like New Zealand - are considering decriminalising or legalising its use.
Professor Hancox said there was still a lot to learn about cannabis, but there was sufficient evidence to show smoking was not harmless to the lungs.
"Our findings were that it does cause lung damage, although the effects are not quite the same as tobacco."
In heavy users - smoking at least a joint a day over a long period of time - it could lead to a form of lung destruction known as "bong lung".
"That involves very regular use over many years."
But even in those smoking less than a joint a day research suggested airways could become inflamed and irritated, and lead to issues such as bronchitis - an inflammation of the lining of your bronchial tubes, which carry air to and from your lungs.
One study they looked at indicated one joint could be the equivalent of smoking 20 cigarettes in terms of impacts on the lungs.
A combination of smoking both cannabis and tobacco was likely to result in even poorer health outcomes.
"Many people smoke both cannabis and tobacco and are likely to get the worst of both substances," Hancox said.
However, as cannabis has been an illegal substance, and most users also smoke tobacco, it was difficult to carry out research on its direct impact on the lungs, Dr Kathryn Gracie from Waikato Hospital's Respiratory Department said.
"Perhaps, most importantly, the individuals who are extremely heavy users of cannabis may not be well represented in the existing epidemiological research.
"Most case reports of cannabis-related destructive lung disease document very heavy cannabis consumption.
"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 could also increase the risk of lung cancer, although there was not enough evidence to be sure yet, Gracie said.
The researchers said with the referendum on legalising cannabis for recreational use this year they would like to see more discussion about the respiratory health impacts.
"We are not here to say we are for or against legalisation, but just that smoking it does have toxic effects on the lungs, and that we would not want to see an increase in a large amount of people smoking heavily over a long period," Hancox said.
Other forms of consuming cannabis such as edibles would obviously be less damaging to lungs, however vaping, where the substance is not burned but still inhaled, had not been properly studied.
"The main issue for the lungs here is the burning, so obviously if you don't smoke it, it will do less damage to your lungs, although there are other health issues associated with cannabis to consider," Hancox said.