Cognition and motor control as a function of Delta9-THC concentration in serum and oral fluid: limits of impairment

Drug Alcohol Depend. 2006 Nov 8;85(2):114-22. doi: 10.1016/j.drugalcdep.2006.03.015. Epub 2006 May 24.


Cannabis use has been associated with increased risk of becoming involved in traffic accidents; however, the relation between THC concentration and driver impairment is relatively obscure. The present study was designed to define performance impairment as a function of THC in serum and oral fluid in order to provide a scientific framework to the development of per se limits for driving under the influence of cannabis. Twenty recreational users of cannabis participated in a double-blind, placebo-controlled, three-way cross-over study. Subjects were administered single doses of 0, 250 and 500 microg/kg THC by smoking. Performance tests measuring skills related to driving were conducted at regular intervals between 15 min and 6h post smoking and included measures of perceptual-motor control (Critical tracking task), motor impulsivity (Stop signal task) and cognitive function (Tower of London). Blood and oral fluid were collected throughout testing. Results showed a strong and linear relation between THC in serum and oral fluid. Linear relations between magnitude of performance impairment and THC in oral fluid and serum, however, were low. A more promising way to define threshold levels of impairment was found by comparing the proportion of observations showing impairment or no impairment as a function of THC concentration. The proportion of observations showing impairment progressively increased as a function of serum THC in every task. Binomial tests showed an initial and significant shift toward impairment in the Critical tracking task for serum THC concentrations between 2 and 5 ng/ml. At concentrations between 5 and 10 ng/ml approximately 75-90% of the observations were indicative of significant impairment in every performance test. At THC concentrations >30 ng/ml the proportion of observations indicative of significant impairment increased to a full 100% in every performance tests. It is concluded that serum THC concentrations between 2 and 5 ng/ml establish the lower and upper range of a THC limit for impairment.

Publication types

  • Comparative Study
  • Randomized Controlled Trial
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Automobile Driving / psychology
  • Cognition / drug effects*
  • Cross-Over Studies
  • Dose-Response Relationship, Drug
  • Double-Blind Method
  • Dronabinol / adverse effects
  • Dronabinol / blood*
  • Female
  • Humans
  • Impulsive Behavior / blood
  • Impulsive Behavior / chemically induced
  • Impulsive Behavior / psychology
  • Male
  • Marijuana Smoking / adverse effects
  • Marijuana Smoking / blood*
  • Marijuana Smoking / psychology
  • Metabolic Clearance Rate / physiology
  • Psychomotor Performance / drug effects*
  • Pursuit, Smooth / drug effects
  • Reaction Time / drug effects
  • Risk Factors
  • Saliva / metabolism


  • Dronabinol