Faster Responses Under Cocaine Did Not Mean Better Impulse Control
Response speed and response accuracy can move in opposite directions. In a controlled study of heavy cannabis users, cocaine improved some psychomotor measures while increasing errors on impulsivity tasks; THC also increased response errors.
The drugs were tested on separate occasions
Sixty-one heavy cannabis users with a history of cocaine use participated in a blinded, placebo-controlled three-way crossover. They received single-drug THC, cocaine and placebo conditions and completed psychomotor and impulse-control tasks. This was not a trial of simultaneous THC–cocaine administration.
The outcome depended on the task
The abstract reports psychomotor impairment after THC and increased impulsivity-task errors. Cocaine improved psychomotor performance and shortened response time in impulsivity tasks while also increasing errors. Describing only the faster response would omit the accompanying reduction in accuracy.
Heavy use did not remove every THC effect
The participants remained sensitive to THC across several neuropsychological domains. Comparisons with previously studied occasional users were historical comparisons, not a randomized comparison of heavy and occasional use in this experiment.
The findings do not supply a blood concentration threshold or an individual driving determination. They illustrate why a performance claim should specify whether it concerns speed, accuracy, inhibition or a composite measure. Better results on one task cannot establish generally improved function, and familiarity with cannabis does not demonstrate immunity to all of its acute effects.
Related: interpreting cannabis tolerance claims.
Research cited
van Wel JH; Kuypers KP; Theunissen EL; Toennes SW; Spronk DB; Verkes RJ; Ramaekers JG. Single doses of THC and cocaine decrease proficiency of impulse control in heavy cannabis users. British journal of pharmacology (2013). doi:10.1111/bph.12425. Findings discussed from the study abstract.
