Some would debate whether Zoe was addicted to marijuana, but there’s no question she was dependent on it. She had all the signs of chemical dependency, including the physical and emotional irritability of withdrawal. Studies in rats show that if they get used to a chronic dose of tetrahydrocannabinol (THC) — the active compound in marijuana — and then are deprived of it, the brain floods the system with corticotropin-releasing factor, which activates the amygdala and thus the entire stress system. The rodents experience shaking, tremors, and twitchy movements that peak about forty-eight hours after the last dose. Indeed, Zoe felt like a rat in a cage when she went through detox: along with the physical symptoms, the shutdown of the dopamine system brought on intense feelings of depression and anxiety. The way exercise blunts the symptoms of withdrawal is by calming the amygdala and boosting dopamine.