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Background: Cannabis is the most widely used illegal drug worldwide. Long term use of cannabis is known to cause chronic bronchitis and airflow obstruction, however the frequency of macroscopic emphysema, the dose-response relationship and the dose equivalence of cannabis with tobacco has not been determined.

Methods: A convenience sample of adults from the Greater Wellington Region was recruited into four smoking groups; cannabis only, tobacco only, combined cannabis and tobacco and non-smokers of either substance. Their respiratory status was assessed using high resolution CT scanning, pulmonary function tests and a respiratory and smoking questionnaire. Associations between respiratory status and cannabis use were examined by analysis of covariance and logistic regression.

Results: A total of 339 subjects were recruited into the four groups. A dose-response relationship was found between cannabis smoking and reduced FEV1/FVC and sGaw, and increased TLC. For measures of airflow obstruction, one cannabis joint had a similar effect to between 2.5 and 6 tobacco cigarettes. Cannabis smoking was associated with decreased lung density on HRCT scans. Macroscopic emphysema was detected in 1/75 (1.3%), 15/92 (16.3%), 17/91 (18.9%) and 0/81 subjects in the cannabis only, combined cannabis and tobacco, tobacco alone and non-smoking groups respectively.

Conclusions: Smoking cannabis was associated with a dose-related impairment of large airways function resulting in airflow obstruction and hyperinflation. In contrast, cannabis smoking was seldom associated with macroscopic emphysema. The 1:2.5 to 6 dose equivalence between cannabis joints and tobacco cigarettes for adverse effects on lung function is of major public health significance.