Early exposure to cannabinoids in adolescent rodents decreases the reactivity of brain dopamine reward centers later in adulthood.28 To the extent that these findings generalize to humans, this could help explain early marijuana initiates’ increased vulnerability for drug abuse and addiction to other substances of abuse later in life that has been reported by most epidemiological studies.29 It is also consistent with animal experiments showing THC’s ability to "prime" the brain for enhanced responses to other drugs.30 For example, rats previously administered THC show heightened behavioral response not only when further exposed to THC but also when exposed to other drugs such as morphine—a phenomenon called cross-sensitization.3