Drug Policy

Unlike urine, blood test results can give a useful indicator of whether one is impaired by marijuana. Studies have shown that high THC blood levels are correlated with impaired driving. An expert panel review of scientific studies on driving under the influence of cannabis concluded that THC levels above 7 - 10 ng/ml in plasma or 3.5 - 5 ng/ml in blood indicate likely impairment [Grotenhermen]. The same review found no increased driving hazard at low levels of THC. Despite the fact that accident studies have repeatedly failed to find evidence of increased driving risk at low levels (1 or 2 ng in blood) of THC, numerous states and foreign countries have enacted "zero-tolerance" laws, treating any non-zero trace of THC as legal evidence for driving under the influence.