A New Zealand study has found smoking cannabis damages the lungs but in a different way to tobacco.
Study co-author Professor Bob Hancox said until recently, it was assumed that cannabis would have similar effects to tobacco, but new University of Otago research has found this is not the case.
"The research found that prolonged cannabis use led to over-inflated lungs and increased the resistance to airflow to a greater extent than tobacco," Hancox said.
"It was also found that cannabis use may also impair the ability of the lungs to extract oxygen from the breath. This is a known consequence of smoking tobacco, but has not been demonstrated with cannabis until now."
About 75 percent of the study members had used cannabis at some stage in their lives but little research has been done into the effects it does on the lungs.
"This is because it has been difficult to study a drug that remains illegal in most parts of the world," Hancox said.
Respiratory doctors recognise that some patients who smoke a lot of cannabis have a distinct form of lung damage - a very severe form of emphysema that is sometimes called bong lung - but little is actually known about this condition.
Hancox said that the latest findings suggested that heavy cannabis use leads to over-inflated lungs, increased airway resistance and impaired oxygen extraction may reflect the early signs of this disease.
"The latest Dunedin study findings support observations from other research that cannabis has different effects to tobacco," he said.
"The Dunedin results extend these findings with more complete measures of cannabis use and a full assessment of lung function using eleven different measures.
"Of course, people who smoke both cannabis and tobacco are likely to suffer lung damage from both substances."
"Why cannabis harms the lungs in different ways to tobacco is not known. Given the widespread and increasing use of cannabis globally and the trend to decriminalisation, understanding its respiratory effects is a priority."
The findings are from a long-running Dunedin Multidisciplinary Health and Development Study, which has documented cannabis use and measured lung function throughout adult life up to the age of 45 in more than 1000 individuals born in Dunedin in 1972/73.
Hancox said the Dunedin Study may have the world's most complete data on lifetime cannabis use and lung function in a large population sample.
