Psychomotor Impairing Effects Of Cannabis Are Nominal In Experienced Users, Study Says - NORML

November 18, 2010 - Maastricht, The Netherlands

Maastricht, The Netherlands: Experienced marijuana consumers become tolerant to the substance's impairing potential on psychomotor skills, according to clinical trial data published online in the journal Psychopharmacology.

Investigators at Maastricht University in The Netherlands and Goethe University in Germany assessed the neurocognitive effects of cannabis and alcohol in 21 experienced marijuana consumers (defined as smoking more than four days per week.) Subjects completed various driving simulator performance tests, including measures of perceptual motor control (critical tracking test), dual task processing (divided-attention task), motor inhibition (stop-signal task), and cognition (Tower of London).

Researchers determined that alcohol dosing significantly impaired subjects' critical tracking, divided attention, and stop-signal performance. By contrast, investigators reported that smoking THC cigarettes "generally did not affect task performance."

"THC did not affect performance of heavy cannabis users in the critical tracking task, the stop-signal task, and the Tower of London," authors wrote. "These tasks have previously been shown to be very sensitive to the impairing potential of THC when administered to infrequent cannabis (users). The lack of THC on these tasks basically confirms the previous notions that heavy cannabis users can develop tolerance to behaviorally impairing effects of THC."

Authors did report that subjects' performance on the divided-attention task was affected by both THC and alcohol, and their combination.

Investigators further reported that heavy marijuana use did not produce cross-tolerance to the impairing potential of alcohol alone or the synergistic effects of alcohol and cannabis when used concurrently.

A previously published study from July also found that experienced cannabis consumers overall performance accuracy on episodic memory and working memory tasks was not significantly altered by marijuana. Researchers in that study concluded: "This pattern of effects is consistent with results previously reported by other researchers studying the acute effects of marijuana on cognitive performance of regular users. ... The observation that frequent users' response accuracy is not altered after marijuana smoking to the same extent it is for infrequent users ... suggests that near-daily marijuana smokers may have developed tolerance to some marijuana-related behavioral effects."

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. Full text of the study, "Tolerance and cross-tolerance to neurocognitive effects of THC and alcohol in heavy cannabis users," appears online in the journal Psychopharmacology. For additional information regarding marijuana and psychomotor skills, please see NORML's white paper, "Cannabis and Driving: A Scientific and Rational Review" here: http://www.norml.org/index.cfm?Group_ID=7459.