Because cannabis is illegal in Germany but not in The Netherlands, many German cannabis users regularly cross the Dutch frontier to buy small amounts of cannabis from 'coffee shops' just over the border. Regular checkpoints are used by the German police, who take blood specimens from any drivers whom they suspect of an offence under the influence of drugs. The consumption of cannabinoids is known to impair driving behaviour. This paper describes the chemical analyses used to analyse blood serum in such cases, and presents some results for samples that were positive for cannabinoids and negative for other drugs. The authors were able to correlate impairments and test failures, reported by the police and/or doctors, with serum concentrations of THC and its metabolites. Because of its very long biological half-life of eight days, free and conjugated THC-carbonic acid accumulates in the serum of regular hashish smokers. The sample of smokers investigated was divided into moderate, heavy and chronic cannabis users. Although the most important guideline for measuring cannabis effects is serum THC concentration, it remains difficult to use low serum THC concentrations to estimate driving impairment after moderate cannabis smoking.
Abstract