In addition, PET studies have shown that the pharmacokinetic profiles of drugs are crucial for their reinforcing effects. In particular, the speed at which drugs enter the human brain is associated with its rewarding effects. By labeling various drugs with positron emitters, it has been possible to monitor their pharmacokinetics in the human brain. These studies have shown that, after intravenous administration, nicotine reaches peak levels in 2–3 min, cocaine in 4–6 min, and methamphetamine in 10–15 min. In addition, at least for stimulant drugs, the fast drug uptake is associated with the “high” described by drug users (Figure 1) (20, 21). The various routes of drug administration (oral, smoked, injected) influence the speed at which drugs are delivered into the brain, which in turn affects their rewarding effects. For example, when cocaine enters the brain rapidly (smoked or intravenous administration) it elicits a more intense “high” than when it enters the brain more slowly (snorted) even when the cocaine doses given to participants are adjusted to produce the same levels of DA transporter blockade in the brain, which is
