Many studies have found a link between legal and illegal drug use and psychologic disorders. Drug use has been found to be related to substance use disorders, including nicotine dependence, anxiety, and affective disturbances. In addition, drug users have higher rates and more severe levels of psychologic impairment than persons who do not use drugs. Despite these results, it is still unclear whether drug use precedes the development of psychiatric disorders or occurs as a result. Previous studies on this issue have been cross-sectional and, therefore, provide only limited information. Brook and associates studied the effects of drug use and substance use during childhood, adolescence, and the early 20s on the development of major depressive disorders, alcohol dependence, and substance use disorders among persons in their late 20s.

The study was a longitudinal design that followed persons at approximately 14, 16, 22, and 27 years of age from a community-based sample. The participants were randomly selected from the population census data of two counties. Households with at least one child qualified for the study, and if more than one child lived within the household, the participating child was randomly selected. Trained interviewers collected data in each participant’s home. Assessments were made for previous major depressive disorders and substance use disorders during the initial interview. At each subsequent visit, the interviewers collected data on a history of any substance use, including tobacco and illegal substances. At the final interview, the participants were assessed for psychiatric disorders using the University of Michigan Composite International Diagnostic Interview.

Frequency of tobacco use was associated with an increased risk of alcohol dependency and substance abuse, but not with major depressive disorder. Early alcohol and substance use significantly predicted later major depressive disorders, alcohol dependency, and substance abuse disorder. The results were still valid when the researchers controlled for other variables, such as age, sex, parental education level, family income, and previous psychiatric disorders, during the analysis.

The authors conclude that early drug use is associated with later psychiatric disorders. They state that drug use precedes major depressive disorders and alcohol dependency, and does not occur as a result of these disorders. They add that certain types of drug use during childhood and adolescence should not be treated as benign conditions that may be outgrown. This is important to consider when physicians are planning prevention and treatment interventions for these patients.