Further analysis was undertaken in which the risk of lung cancer
was assessed for cannabis use up to 5 yrs prior to the diagnosis.
This approach was based on evidence that in most cases of lung
cancer, the duration between malignant change and diagnosis is
.5 yrs [26]. When cannabis use in the 5 yrs prior to diagnosis for
the cases (and prior to the reference date for the controls) was
excluded from the analysis the strength of the association was
maintained, as would be expected if a causal association existed,
with a 10% increase in risk for each joint-yr of exposure