Chapter Three – Epidemiology of Drug Abuse: Building Blocks for Etiologic Research

The epidemiology of substance use (SU) and substance use disorders (SUDs) provides essential information for understanding the etiology of these complex conditions. After peaking in the late 1970s, overall SU rates declined during the 1980s and then rose again during the first half of the 1990s. Since the mid/late 1990s, overall SU rates declined slowly or remained generally stable. Within this overall pattern, many specific drug epidemics were observed, including crack cocaine, MDMA (ecstasy), methamphetamine, and, recently, prescription-type drug epidemics. Onset of both SU and SUDs is typically during adolescence and rates of SU generally peak in late adolescence and early adulthood. Early onset is associated with higher rates of SUDs and worse prognosis. Patterns of SU shift across different gender, race/ethnicity and age groups, with inhalants more commonly abused by children and early adolescents than by older adolescents and adults. Polysubstance use is a typical pattern of SU, especially among youth. Strong associations of SU/SUD and psychiatric illnesses are the rule rather than the exception, especially for antisocial personality disorder (and other personality disorders) and bipolar disorder. SU and SUDs result from a complex interplay of individual-level, environmental, and developmental factors. Numerous studies have found that heritability of SUD rises with age and with progression of disorders, suggesting that genetic factors play an increasing role as individuals pass through later adolescence into adulthood and as their SU advances to heavier use and addiction. Heritability studies have also shown that much of the genetic risk is nonspecific with regard to choice of substance. In addition to genetic vulnerability to SUDs, there is evidence for genetic predisposition to a constellation of externalizing behaviors across development (e.g. conduct disorder and attention deficit hyperactivity disorder). In addition to and often interacting with genetic risk, multiple environmental factors influence the onset and progression of drug use and drug-use disorders, including: adverse family circumstances, early stress and child abuse, parenting practices (especially parental monitoring), religiosity, prenatal exposures, peer factors, school factors, and neighborhood factors (including drug availability). Key observations with potential relevance for neuroscience research include: (1) SUDs frequently overlap with other psychiatric disorders, (2) polysubstance use is a typical pattern, rather than use of single substances, (3) the environment plays a key role in shaping vulnerability and resilience factors, and (4) developmental issues are central to understanding SU and SUDs, especially in terms of early risk factor exposures and timing of drug initiation.