Heavy Cannabis Use Did Not Prevent THC-Related Stopping Deficits in a Human Trial

Heavy cannabis users were less affected on several tasks in a small controlled trial, but still showed a THC-related deficit in stopping an initiated response. That exception matters when discussing tolerance.

The study compared two use-history groups

Twelve occasional and twelve heavy users received smoked THC and placebo in a blinded design. Testing included critical tracking, divided attention, a stop-signal task and a planning task over the post-smoking period.

Effects were task-dependent

The abstract reports impairment in occasional users on tracking, divided attention and stopping. Heavy users did not show the same pattern across all tasks, but their stop reaction time increased under THC, particularly at higher measured THC concentrations.

An exception can defeat a global claim

Reduced sensitivity in tracking does not establish intact motor inhibition. These functions place different demands on performance, and a user may adapt to one more than another.

Use history was not randomly assigned, and each group was small. The findings do not prove that every between-group difference was caused by acquired tolerance rather than other group characteristics. Nor does the reported concentration relationship provide a validated legal or clinical cutoff. The relevant conclusion is that frequent use did not eliminate every measured acute THC effect.

Related: interpreting cannabis tolerance claims.

Research cited

Ramaekers JG; Kauert G; Theunissen EL; Toennes SW; Moeller MR. Neurocognitive performance during acute THC intoxication in heavy and occasional cannabis users. Journal of psychopharmacology (Oxford, England) (2009). doi:10.1177/0269881108092393. Findings discussed from the study abstract.