• Claudine C Hunault, Tjeert T Mensinga, Koen B E Böcker, C Maarten A Schipper, Maaike Kruidenier, Marianne E C Leenders, Irma de Vries, and Jan Meulenbelt.
• National Poisons Information Center, National Institute for Public Health and the Environment, Bilthoven, The Netherlands. claudine.hunault@rivm.nl
• Psychopharmacology (Berl.). 2009 May 1; 204 (1): 85-94.

RationaleDelta(9)-Tetrahydrocannabinol (THC) is the main active constituent of cannabis. In recent years, the average THC content of some cannabis cigarettes has increased up to approximately 60 mg per cigarette (20% THC cigarettes). Acute cognitive and psychomotor effects of THC among recreational users after smoking cannabis cigarettes containing such high doses are unknown.ObjectivesThe objective of this study was to study the dose-effect relationship between the THC dose contained in cannabis cigarettes and cognitive and psychomotor effects for THC doses up to 69.4 mg (23%).Materials And MethodsThis double-blind, placebo-controlled, randomised, four-way cross-over study included 24 non-daily male cannabis users (two to nine cannabis cigarettes per month). Participants smoked four cannabis cigarettes containing 0, 29.3, 49.1 and 69.4 mg THC on four exposure days.ResultsThe THC dose in smoked cannabis was linearly associated with a slower response time in all tasks (simple reaction time, visuo-spatial selective attention, sustained attention, divided attention and short-term memory tasks) and motor control impairment in the motor control task. The number of errors increased significantly with increasing doses in the short-term memory and the sustained attention tasks. Some participants showed no impairment in motor control even at THC serum concentrations higher than 40 ng/mL. High feeling and drowsiness differed significantly between treatments.ConclusionsResponse time slowed down and motor control worsened, both linearly, with increasing THC doses. Consequently, cannabis with high THC concentrations may be a concern for public health and safety if cannabis smokers are unable to titrate to a high feeling corresponding to a desired plasma THC level.

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