Tolerance (clinical)

Tolerance is a pharmacological state in which repeated exposure to a drug produces diminished response at a given dose, requiring higher doses to achieve the prior effect. For THC, tolerance is driven principally by homologous CB1 receptor desensitization (β-arrestin-mediated uncoupling from Gi/o signaling) and downregulation (internalization and reduced surface receptor density). Human PET imaging has directly demonstrated this neuroadaptation: Hirvonen et al. (2012, Molecular Psychiatry) using [18F]FMPEP-d2 found ~20% lower cortical CB1 receptor availability in chronic daily smokers versus controls, with reversal to normal after approximately 4 weeks of monitored abstinence. D'Souza et al. (2016, Biological Psychiatry: CNNI) using [11C]OMAR showed that CB1 upregulation begins within ~48 hours of cessation and continues over weeks. Tolerance develops unevenly across THC's effects (e.g., faster to cardiovascular and cognitive effects than to appetite stimulation). This clinical/pharmacological definition is distinct from the lay "tolerance break" or "T-break" consumption concept. → See also: CB1 Receptor, Dependence, Withdrawal, Tolerance (Part 5 consumption sense).