Further analysis was undertaken in which the risk of lung cancer
was assessed for cannabis use up to 5 yrs prior to the diagnosis.
The present findings are consistent with the three North African case–control studies [16–18] that have reported a six- to eight-fold increased risk of lung cancer with cannabis smoking, although the lack of detailed smoking histories and the custom of mixing cannabis with tobacco may have contributed to the risks observed. In contrast, a large case–control study from California, USA, with an estimated participation rate in cases of only 39% reported no association between cannabis use and
lung cancer [19]. The controls in the Californian study were matched for neighbourhood, which may have made detection of an association less likely, as cannabis use is likely to be similar within distinct neighbourhoods. This may have contributed to the higher rates of cannabis use in the controls
than previously reported from California [38]. Also, African- American people were less likely to have participated than other ethnic groups, suggesting significant selection bias may have occurred. A positive relationship between cannabis use and cigarette smoking was observed in the controls, but the opposite relationship was present in the lung cancer cases, which also suggests selection bias or differential reporting between cases and controls. The practice of mixing tobacco with cannabis in the joint appears to vary between populations and may contribute to the geographical variation in risk of
lung cancer from cannabis use observed.