You are not permitted to download, save or email this image. Visit image gallery to purchase the image.
Respiratory specialists from Dunedin and Waikato Hospitals have reviewed research from New Zealand and overseas involving thousands of people.
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.
Prof Hancox said more attention should be paid to adverse physical effects on the lungs during the debate over legalising cannabis
"You can get quite a nasty bronchitis, even if you’re using it once a day.
"We’ve got to keep the physical effects on the lung in mind when we’re considering what to do about cannabis, as well as mental health and social effects," he said yesterday.
Cannabis was harmful to the lungs, but in a different way to tobacco, causing significant respiratory symptoms such as bronchitis, the review of research on the effects of smoking by respiratory specialists Prof Hancox, of the Otago preventive and social medicine department and Dr Kathryn Gracie, of Waikato Hospital’s respiratory department, found.
The potential for adverse effects on respiratory health from smoking cannabis had had much less attention than the social and mental health effects in recent discussion, Prof Hancox said.
"I haven’t heard much talk about what the effects are on the lung and other health aspects," he said.
"We believe policies around the liberalisation of cannabis should consider the potential impacts on the lungs."
If cannabis availability was liberalised it was likely that patterns of cannabis use would change, with resulting health consequences, he said.
"Many people smoke both cannabis and tobacco and are likely to get the worst of both substances."
Cannabis was the second-most commonly smoked substance after tobacco and the most widely-used illicit drug world-wide.
Although cannabis remained illegal in most countries, many nations, like New Zealand, were considering decriminalising or legalising its use.
Dr Gracie said because cannabis was an illegal and unregulated substance and most cannabis users also smoked tobacco, it was difficult to separate the effects of smoking cannabis.
For these reasons it had also been difficult to carry out research on cannabis and its direct impact on the lungs.
Furthermore, people who were extremely heavy users of cannabis might not be well represented in the existing research on cannabis use.
Nevertheless, there was sufficient evidence that cannabis use could "damage both the airways and the lungs", Dr Gracie said.