The rewarding effect of drugs of abuse is the main reason why humans use them and laboratory animals self-administer them (1). However, most individuals using drugs never escalate to uncontrollable levels of drug use and only a relatively small percentage does (approximately 10–20%). These are the individuals who fit diagnostic criteria (as per DSM IV) for substance abuse or dependence (compulsive drug use with loss of control over intake despite adverse consequences) (2). Who becomes addicted (term used in lieu of the dependence term used by DSM IV to avoid confusion with physical dependence) and who does not are strongly influenced by genetic (50% of risk), developmental (risk is higher in adolescence than adulthood), and environmental factors (drug access, stress), as well as the type of drug (some drugs produce addiction faster than others, e.g., methamphetamine versus marijuana) (2).
