Investigations of urine and blood before regranting driving licences to consumers of cannabis.

Cannabis users take an exceptional part of drug consumers because the aptitude for driving motor vehicles is considered to be impaired only if habitual consumption is to be assumed. Urine is a useful material for drug testing. But urine samples can be manipulated not only by drinking high amounts of water. If information about the frequency of consumption is necessary, blood should be analysed because the concentration of the THC-metabolite in urine depends not only on the concentration in blood and the creatinine content but also on other factors.