Tolerance (consumption)

Tolerance is the progressive reduction in the magnitude of subjective, cognitive, psychomotor, cardiovascular, and endocrine effects of THC with repeated dosing, requiring larger doses for equivalent effect. It develops over days to weeks of regular (typically daily or near-daily) use. The principal mechanism is pharmacodynamic: chronic CB1 agonism drives receptor desensitization (uncoupling from Gi/o) and downregulation (internalization and reduced surface density), most robustly in cortical regions. Hirvonen et al. (2012 Mol Psychiatry) used [¹⁸F]FMPEP-d2 PET to show ~20% cortical CB1R downregulation in chronic daily smokers, with receptor density returning to control levels after ~4 weeks monitored abstinence. D'Souza et al. (2008) demonstrated blunted psychotomimetic, amnestic, and cortisol responses to IV THC in frequent versus infrequent users, with euphoria preserved. Ramaekers (2011, 2016) showed partial, task-specific behavioral tolerance. Tolerance is neuroadaptive, distinct from dependence/withdrawal (Part 9). Citations: Hirvonen J et al. Mol Psychiatry. 2012;17:642-649; D'Souza DC et al. Neuropsychopharmacology. 2008;33:2505-2516; Colizzi M, Bhattacharyya S. Neurosci Biobehav Rev. 2018;93:1-25.