Differential Effects of Cannabis and Tobacco on Lung Function in Mid-Adult Life.

RATIONALE Evidence suggests that the effects of smoking cannabis on lung function are different to tobacco. However, long-term follow-up data are scarce, and mostly based on young adults. OBJECTIVE To assess the effects of cannabis and tobacco on lung function in mid-adult life. METHODS Cannabis and tobacco use were reported at ages 18, 21, 26, 32, 38, and 45 years in a population-based cohort study of 1037 participants. Spirometry, plethysmography, and carbon monoxide transfer factor were measured at age 45. Associations between lung function and cannabis use were adjusted for tobacco use. MEASUREMENTS AND MAIN RESULTS Data were available from 881 (88%) of 997 surviving participants. Cumulative cannabis use was associated with lower Forced Expiratory Volume in one second to Forced Vital Capacity ratios, due to a tendency towards higher Forced Vital Capacities. Cannabis use was also associated with higher total lung capacity, functional residual capacity, residual volume, and alveolar volume along with lower mid-expiratory flows, airway conductance, and transfer factor. Quitting regular cannabis use between assessments was not associated with changes in spirometry. CONCLUSIONS Cannabis use is associated with higher lung volumes suggesting hyperinflation. There is evidence of increased large-airways resistance and lower mid-expiratory airflow, but impairment of Forced Expiratory Volume in one second to Forced Vital Capacity ratio is due to higher Vital Capacities. This pattern of effects is different to those of tobacco. We provide the first evidence that lifetime cannabis use may be associated with impairment of gas transfer.