Scientists at the Department for Forensic and Traffic Medicine of the University of Heidelberg, Germany, investigated the effects of cannabis on driving related functions in a 28 year old man with attention-deficit/hyperactivity disorder (ADHD). He had violated traffic regulations several times in recent years and his driving licence was revoked due to driving under the influence of cannabis. He showed abnormal behaviour, seemed to be significantly maladjusted and his concentration was heavily impaired while sober during the first meeting with a psychologist. He was allowed to perform driving related tests under the influence of the cannabis compound dronabinol (THC), which his doctor had prescribed him to treat his symptoms. The examiner expected that he was not able to drive a car under the acute influence of THC.
But at the second visit his behaviour was markedly improved and he performed average and partly above-average in all tests on reaction speed, sustained attention, visual orientation, perception speed and divided attention. A blood sample taken after the tests revealed a high THC concentration of 71 ng/ml in blood serum. He admitted later to have smoked cannabis and not taken dronabinol, because it was too expensive. Researchers noted that "people with ADHD are found to violate traffic regulations, to commit criminal offences and to be involved in traffic accidents more often than the statistical norm" and conclude from their investigation that "it has to be taken into account that in persons with ADHD THC may have atypical and even performance-enhancing effects."
Smoked Marijuana Improved ADHD Driver's Performance by: Strohbeck-Kuehner P, Skopp G, Mattern R. Arch Kriminol 2007;220(1-2):11-9.