title: Driving under the influence of cannabis: a 10-year study of age and gender differences in the concentrations of tetrahydrocannabinol in blood.
abstract: BACKGROUND
  Delta(9)-Tetrahydrocannabinol (THC) is the major psychoactive constituent of cannabis and its various preparations. Increasing use of cannabis for recreational purposes has created a problem for road-traffic safety. This paper compares age, gender and the concentrations of THC in blood of individuals apprehended for driving under the influence of drugs (DUID) in Sweden, where a zero-tolerance law operates.
  
  
  METHODS
  Specimens of blood or urine were subjected to a broad screening analysis by enzyme immunoassay methods. THC positives were verified by analysis of blood by gas chromatography-mass spectrometry (GC-MS) with a deuterium-labelled internal standard (d(3)-THC). All toxicology results were entered into a database (TOXBASE) along with the age and gender of apprehended drivers.
  
  
  RESULTS
  Over a 10-year period (1995-2004), between 18% and 30% of all DUID suspects had measurable amounts of THC in their blood (> 0.3 ng/ml) either alone or together with other drugs. The mean age [+/- standard deviation (SD)] of cannabis users was 33 +/- 9.4 years (range 15-66 years), with a strong predominance of men (94%, P < 0.001). The frequency distribution of THC concentrations (n = 8794) was skewed markedly to the right with mean, median and highest values of 2.1 ng/ml, 1.0 ng/ml and 67 ng/ml, respectively. The THC concentration was less than 1.0 ng/ml in 43% of cases and below 2.0 ng/ml in 61% of cases. The age of offenders was not correlated with the concentration of THC in blood (r = -0.027, P > 0.05). THC concentrations in blood were higher when this was the only psychoactive substance present (n = 1276); mean 3.6 ng/ml, median 2.0 ng/ml compared with multi-drug users; mean 1.8 ng/ml, median 1.0 ng/ml (P < 0.001). In cases with THC as the only drug present the concentration was less than 1.0 ng/ml in 26% and below 2.0 ng/ml in 41% of cases. The high prevalence of men, the average age and the concentrations of THC in blood were similar in users of illicit drugs (non-traffic cases).
  
  
  CONCLUSIONS
  The concentration of THC in blood at the time of driving is probably a great deal higher than at the time of sampling (30-90 minutes later). The notion of enacting science-based concentration limits of THC in blood (e.g. 3-5 ng/ml), as discussed in some quarters, would result in many individuals evading prosecution. Zero-tolerance or limit of quantitation laws are a much more pragmatic way to enforce DUID legislation.
authors.affiliation: Department of Forensic Genetics and Forensic Toxicology, National Board of Forensic Medicine, Sweden. wayne.jones@rmv.se
authors.forename: Alan W
authors.initials: AW
authors.lastname: Jones
authors.forename: Anita
authors.initials: A
authors.lastname: Holmgren
authors.forename: Fredrik C
authors.initials: FC
authors.lastname: Kugelberg
mesh_terms: D000063:Accidents, Traffic; D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001334:Automobile Driving; D013759:Dronabinol / Q000097:blood*; D005260:Female; D006801:Humans; D008297:Male; D008385:Marijuana Smoking / Q000097:blood* / Q000453:epidemiology; D008875:Middle Aged; D012737:Sex Factors; D015813:Substance Abuse Detection / Q000379:methods; D013548:Sweden / Q000453:epidemiology
chemical_list: D013759:Dronabinol