Cannabis, tobacco smoking, and lung function: a cross-sectional observational study in a general practice population.

BACKGROUND Health concerns around cannabis use have focused on the potential relationship with psychosis but the effect of cannabis smoking on respiratory health has received less attention. AIM To investigate the association between tobacco-only smoking compared with tobacco plus cannabis smoking and adverse outcomes in respiratory health and lung function. DESIGN AND SETTING The design was cross-sectional with two groups recruited: cigarette smokers with tobacco pack-years; cannabis smokers with cannabis joint-years. Recruitment occurred in a general practice in Scotland with 12 500 patients. METHOD Exposures measured were tobacco smoking (pack-years) and cannabis smoking (joint-years). Cannabis type (resin, herbal, or both) was recorded by self-report. Respiratory symptoms were recorded using NHANES and MRC questionnaires. Lung function was measured by spirometry (FEV1/FVC ratio). RESULTS Participants consisted of 500 individuals (242 males). Mean age of tobacco-only smokers was 45 years; median tobacco exposure was 25 pack-years. Mean age of cannabis and tobacco smokers was 37 years; median tobacco exposure was 19 pack-years, rising to 22.5 when tobacco smoked with cannabis. Although tobacco and cannabis use were associated with increased reporting of respiratory symptoms, this was higher among those who also smoked cannabis. Both tobacco and cannabis users had evidence of impaired lung function but, in fully adjusted analyses, each additional joint-year of cannabis use was associated with a 0.3% (95% confidence interval = 0.0 to 0.5) increase in prevalence of chronic obstructive pulmonary disease. CONCLUSION In adults who predominantly smoked resin cannabis mixed with tobacco, additional adverse effects were observed on respiratory health relating to cannabis use.