Cannabis and Road Crashes: A Close Look at the Best Epidemiological Evidence

A literature search identified eleven epidemiological studies of the relationship between the prior use of cannabis and crashing. The studies were scrutinised to investigate potential bias. Many of the studies were found to be affected by biases that would exaggerate the apparent effect of cannabis. Accounting for the biases, it is concluded that, if cannabis does increase the risk of crashing, the increase is unlikely to exceed 30% (equivalent to driving with a BAC of below 0.05). Even the null hypothesis of no increase cannot be rejected.