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Smoking cannabis ‘worse for lungs than tobacco’

Smoking cannabis is linked to more respiratory problems than smoking tobacco, according to a UK study in general practice.

The study, published in the British Journal of General Practice, included 500 smokers, of whom 248 were long-term users of tobacco only and 252 were users of both cannabis and tobacco.

The cannabis smokers generally reported more respiratory symptoms, in particular bringing up phlegm, and suffering from wheezing or whistling, and were slightly more likely to have COPD – as indicated by an FEV1/FVC lower than 70% on spirometry.

Even after taking into account the risk from the tobacco they smoked in joints and from also smoking cigarettes, the cannabis users still had a significantly higher rate of symptoms such as bringing up phlegm – the rate of which was increased by 0.4% with each additional joint-year of cannabis use.

Similarly, after accounting for the tobacco-related risk, each additional joint-year of cannabis use was associated with a 0.3% greater prevalence of COPD.

Dr Roy Robertson, from the University of Edinburgh, and colleagues concluded: ‘In a general practice-based sample of established adult tobacco and cannabis users, cannabis use… was associated with greater reporting of respiratory symptoms. In particular, the slightly increased presence of cough, wheeze, and sputum production are more likely in cannabis smokers.

‘In addition, it was also associated with objective evidence of COPD.’

They added: ‘Part of the adverse effect of cannabis is likely attributable to the tobacco included in the cannabis joint; however even after this additional tobacco smoked with cannabis was taken into account, the effects of cannabis on increased respiratory symptoms and increased prevalence of spirometric COPD were still apparent.’

Br J Gen Pr 2014; available online 26 January

Readers' comments (1)

  • The whole premise of this study is absurd. How can you determine the effect of cannabis on lungs if all your subjects are tobacco smokers?

    The largest case control study of its type, Tashkin et al 2006, directly contradicts this study. Pletcher et al 2012, a 20 year longitudinal study also reaches exactly the opposite conclusion.

    Only last month the journal of the American Thoracic Society published another study that concludes "cannabis exposure and inhalation were not associated with negative spirometry changes, just FEV1 (forced expiratory volume) declines or problems with small airways disease. Additionally, the researchers found that vaporizing marijuana users reported even fewer issues than those who inhaled combusted marijuana."

    I'm afraid that such a badly designed study is incredible when set against these far larger and better conceived analyses.

    Interestingly, the Tashkin study actually suggests that cannabis might provide a protective effect against the ravages of tobacco.

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