Dangers of dagga
"We know that smoke from tobacco and other entities – including burning wood in your fireplace – causes chronic bronchitis, so it's not at all surprising they found chronic bronchitis in prior marijuana research," said Dr Norman Edelman, senior scientific adviser to the American Lung Association.
The new study was published in the Annals of Internal Medicine.
Edelman said he's concerned that heavy marijuana use could lead to larger health problems for those who develop chronic bronchitis.
Half of tobacco smokers get COPD
"You would worry about people being more susceptible to pneumonia, and of course, the end result of chronic bronchitis, if it persists long enough and is severe enough, is what we call COPD – chronic obstructive pulmonary disease," Edelman said.
About half of tobacco smokers get COPD, he said. "It will be interesting to see what percentage of regular marijuana smokers get COPD," he added.
Paul Armentano, deputy director of NORML, a group supporting reform of marijuana laws, said the study findings are "consistent with prior data".
"It is hardly surprising that the habitual inhalation of combustive smoke may be associated with specific, though generally mild respiratory symptoms, like cough," he said.
Tobacco still more dangerous
"However, unlike the inhalation of tobacco smoke, cannabis smoke exposure – even long-term – is not associated with the kind of serious respiratory effects that are often identified with long-term tobacco use, such as COPD, emphysema or lung cancer," Armentano said.
About 13% of adults and 21% of young adults are believed to be regular dagga users.
Marijuana legalisation has led to the development of many alternatives to smoking dagga, such as cannabis-infused edibles, oils and concentrates, Armentano said.
For the evidence review, researchers led by Dr Mehrnaz Ghasemiesfe, from the San Francisco VA Medical Center, analysed data from 22 studies of the effects of dagga smoking on lung health.
Analysis of two prospective studies (ones that watch for outcomes such as disease development) found dagga smoking associated with a doubled risk of cough and a nearly quadrupled risk of phlegm, the results showed.
Combined analysis of other studies revealed an increased risk of cough (4.3 times); phlegm (3.4 times); wheezing (2.8 times); and shortness of breath (1.5 times).
Some are concerned that as more US states legalise dagga, more people will develop lung problems.
Years to show effect
"Because some of the worst effects of smoking take years to show effect, it took time until we had established clear and undeniable risks of cancer, heart disease and other major medical problems that were caused by smoking tobacco," said Dr Adam Lackey, chief of thoracic surgery at Staten Island University Hospital.
"I worry that we are looking at a similar situation with marijuana," he said. "People need to realise that we just don't know yet what the long-term effect of marijuana smoking is. This study shows that marijuana smoking certainly isn't totally benign."
At the same time, Edelman, the lung association adviser, doubts dagga will be as harmful as tobacco, simply because it's not smoked as much.
"My guess is that not many marijuana users smoke 20 joints a day, which would be equivalent to a pack a day for a cigarette smoker," he said.
"I don't think the smoke of marijuana is necessarily less toxic than the smoke of tobacco. It's just that in general, people who use marijuana smoke fewer marijuana cigarettes than people who smoke tobacco," Edelman said.
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