Adolescent THC Exposure Created Lasting Tolerance to Cannabis Effects in Adult Monkeys

Nonhuman primates treated daily with THC during adolescence (6 months at low or high doses) and tested approximately one year later as adults showed persistent behavioral tolerance. On a touchscreen attention task (PVT), those with adolescent THC history required higher doses of THC to show impairment compared to vehicle-treated controls, regardless of whether THC was administered intramuscularly or orally.

Female and male adolescent nonhuman primates received daily intramuscular THC (0.32 or 3.2 mg/kg/day) or vehicle for 6 months. After approximately 1 year abstinence, now-adult subjects were trained on a touchscreen Psychomotor Vigilance Task (PVT). Acute THC was administered intramuscularly and orally to assess dose-response relationships on attentional performance.

This is one of the first primate studies to demonstrate that adolescent cannabis exposure creates lasting changes in how the brain responds to cannabinoids, persisting for at least a year after stopping. The finding that tolerance persists through extended abstinence suggests permanent or very long-lasting neuroadaptation during adolescent brain development.

Persistent tolerance has real-world implications: if adolescent cannabis users develop lasting tolerance, they may consume more cannabis as adults to achieve desired effects, increasing exposure to potential harms. This neurobiological evidence supports concerns about adolescent cannabis use even if immediate cognitive effects resolve after quitting.

Relatively small sample typical of primate studies. Only examined attentional performance; other cognitive domains may be affected differently. Cannot determine whether tolerance is permanent or would eventually resolve. Female and male subjects analyzed together rather than separately for sex differences.