The consistent link between common psychiatric problems and substance use has led researchers and practitioners to suggest that by intervening early in adolescence to treat psychiatric disorders, we could reduce substance use problems by late adolescence. However, two key questions need to be answered before we can conclude that intervening on psychiatric problems will be an effective strategy to reduce substance use in adolescence. First, do adolescents who exhibit an increase in their psychiatric problems exhibit a subsequent increase in their substance use? Longitudinal studies provide consistent evidence that youth with higher levels of psychiatric problems are more likely to engage in substance use during adolescence. Etiologic theories to explain this comorbidity are based on causal pathway models, in which conduct disorder, depression, and anxiety result in substance use. Frequent explanations for these relationships are that children and adolescents with conduct disorder gravitate towards social environments that facilitate problem behaviors such as substance use and that drugs like alcohol and marijuana are used to self-medicate or alleviate persistent symptoms of sadness and anxiety. However, existing studies have primarily examined whether youth with higher levels of psychiatric problems are more likely to use and abuse substances , rather than examining whether adolescents tend to increase their level of substance use during periods when their psychiatric problems increase . The latter approach represents a more direct examination of the self-medication hypothesis,mobile vertical grow racks where adolescents increase their substance use in an attempt to manage emerging psychiatric problems. Few longitudinal studies have examined the association between intra-individual changes in mental health problems and substance use.
By examining within-individual change, causal inference is enhanced because selection effects and all factors that vary between individuals are ruled out as potential confounds. It also provides a better indication of whether treating an adolescent’s psychiatric problems could potentially lead to a reduction in his substance use. The second key question is: Are there sensitive periods during adolescence when psychiatric problems play a particularly strong role in shaping substance use? Cerdá and colleagues found no evidence that there was a sensitive period in which acute and chronic psychiatric problems were more strongly related to the onset of alcohol and marijuana use from childhood to late adolescence. Specifically, both recent and cumulative conduct disorder problems were associated with earlier alcohol and marijuana use onset in a cohort of boys followed from ages, whereas cumulative, but not recent, depression problems were associated with earlier alcohol use onset. However, there was no particular age of substance use initiation when psychiatric problems mattered the most. In contrast, Maslowsky and colleagues and Gibbons and colleagues found evidence indicating that early conduct problems were a stronger predictor of alcohol and marijuana use in late adolescence than conduct problems in middle adolescence. However, these three studies focused on between-individual differences in psychiatric problems and substance use. Therefore, it is unclear whether there is a specific developmental period during adolescence when youth are more likely to escalate their drug and alcohol use in response to emerging psychiatric problems. One way to effectively address these two key questions is to use longitudinal data to examine whether youth tend to increase the frequency of their substance use after they experience an increase in their psychiatric problems, and test whether this association changes across development. This type of within-person change analysis eliminates the possibility that time-stable individual differences such as genotype, race/ethnicity, personality traits, family history of psychiatric problems and substance dependence, and parenting problems can explain the association between changes in psychiatric problems and substance use across adolescence. Hence, it controls for all unmeasured time-invariant confounders.
In addition, measured time-varying confounders can also be included as control variables . Using this approach, researchers have shown that change in alcohol abuse or dependence and nicotine dependence in early adulthood predicted change in major depression in a birth cohort in New Zealand. Additionally, increasing frequency of cannabis use was associated with concurrent increasing depression problems in four Australasian birth cohorts . But to our knowledge, no research has used this approach to establish the directionality of the relationship between common psychiatric problems and substance use: that is, to evaluate whether an increase in conduct disorder, depression and anxiety problems leads to a subsequent increase in alcohol and marijuana use; an increase in alcohol and marijuana use leads to a subsequent increase in conduct disorder, depression and anxiety; or a reciprocal relationship exists between psychiatric problems and substance use. Thus, the aims of the present study are to address the following questions: do adolescents experience an increase in the frequency and quantity of their alcohol and marijuana use following an increase in conduct disorder, depression, and anxiety problems? Are there specific periods during adolescence when increases in these mental health problems are more strongly related to escalations in substance use than others? We examine these questions in a longitudinal urban sample of males followed from ages 13 to 19, with yearly measures of psychiatric problems and substance use quantity and frequency. To establish the directionality of these associations, we examine both whether increases in alcohol and marijuana follow increases in conduct disorder, depression, and anxiety, and whether increases in conduct disorder, depression, and anxiety follow increases in alcohol and marijuana use. Data are from the youngest cohort of the Pittsburgh Youth Study. This sample has been described in depth elsewhere.Briefly, participants included first-grade boys enrolled in 31 public schools in Pittsburgh in 1987-1988. A random sample of boys was invited for an initial multi-informant screening. The screen involved assessing the boys’ conduct problems using ratings collected from the parents, teachers, and the boys themselves. Boys whose composite conduct problem scores fell within the upper 30th percentile, together with an approximately equal number of participants randomly selected from the remaining end of the distribution, were selected for longitudinal follow-up .
The sample is predominantly Black and White with 3% Asian, Hispanic, and mixed-race. Participants were assessed annually or semi-annually, depending on the measure, for thirteen years. Caretakers provided informed consent and adolescents provided assent until age and consent thereafter. We restricted analysis to adolescents at ages, as substance use by year was rare at younger ages: 93.9% and 84.5% did not use marijuana or alcohol, respectively, on any occasion between the ages of 7-12. Study procedures were approved by the Institutional Review Boards of the University of Pittsburgh School of Medicine and the Columbia University Mailman School of Public Health. Alcohol and marijuana use were assessed semi-annually by a 16-item Substance Use Scale adapted from the National Youth Survey. Adolescents were queried about timing, quantity, and frequency of alcohol and marijuana use. We defined “marijuana frequency” as the number of occasions of marijuana use in the past year. We defined “alcohol frequency” as the number of occasions of drinking in the past year. We defined “alcohol quantity” as the average number of drinks per occasion in the past year. For phases separated by only 6 months, past-year values were constructed by taking the average of the two semi-annual interviews. Affective, anxiety,vertical cannabis grow systems and conduct problems were measured with items from the Child Behavior Checklist , Teacher Report Form , Youth Self-Report , and Young Adult Self Report from the Achenbach system of assessment.DSM-oriented problem domains were measured with items rated as very consistent with DSM-IV symptoms of affective disorders, anxiety disorders, and conduct disorder by a group of mental health professionals.The scales were administered to caregivers and teachers from age 7 to 16, and youth from age 10 to 19. Items were scored as 0 = not true, 1 = somewhat or sometimes true, and 2 = very true or often true .In order to facilitate comparison across informants, total scores for each scale were converted to t-scores based on age- and gender-specific national norms .The average internal consistency coefficients for the caregiver, teacher, and youth depression scales were 0.82, 0.76, and 0.81, respectively. For the anxiety scales, the internal consistency coefficients for caregiver, teacher and youth scales were 0.72, 0.73, and 0.67, respectively. For the conduct disorder scale, the internal consistency coefficients were 0.91, 0.9, and 0.83 for caregiver, teacher, and youth scales, respectively.These scales have been shown to discriminate between clinic referred adolescents with depressive, anxiety, and conduct disorders and non-referred adolescents. All the scales used have previously shown acceptable concurrent and predictive validity in ROC analyses comparing the scales with official records of offense and delinquency or by assessing discrimination between adolescents referred to psychiatric clinics and non-referred adolescents.Several potential time-varying confounding factors were included in the current study to parse out the effect of psychiatric problems from the constellation of time-varying risk factors that could increase both psychiatric problems and substance use.
The selection of confounders was based on theory and a review of the literature, as detailed below. “Family factors” included changes in socioeconomic status , assessed yearly by applying the Hollingshead Index of Social Status to data provided by the primary caretaker or youth no longer living with family beginning at age 1632; changes in parental supervision/involvement, a 43-question scale concerning caretakers’ knowledge of the youths’ whereabouts, the frequency of joint discussions, planning, and activities, and the amount of time that the youth is unsupervised; positive parenting, a scale measuring perception of frequency of positive responses to youth behavior; parental stress, a 14-item scale measuring perceived stress levels and caretakers’ abilities to cope with stress in the previous month 18; and parental use of physical punishment, drawn from a scale that measures parental discipline strategies. “Peer Variables” consisted of changes in youth peer delinquency and peer substance use, a 15-item scale that corresponds to a self-reported delinquency scale.Analyses were conducted in R version 3.0.2 and 3.0.3. Missing data in the covariates were imputed using R package ‘mice’ for “multivariate imputation by chained equations,” an implementation of fully conditional specified models for imputation. The fully conditional approach differs from the more traditional joint modeling approach by specifying a multivariate imputation model on a variable-by-variable basis. This fully conditional approach is used as an alternative to traditional joint modeling when no suitable multivariate distribution can be found. We imputed 20 datasets, and in subsequent analyses used the R package ‘mitools to pool the results of functions runon the 20 data sets using Rubin’s Rules. We employed quasi-Poisson regression techniques to assess the fixed effects that one-year lagged changes in psychiatric problems had on subsequent changes in alcohol use frequency/quantity and marijuana use frequency from ages 13 to 19. Quasi-Poisson models are an approach to dealing with over-dispersion, which was apparent in initial Poisson models. They use the mean regression function and the variance function from Poisson generalized linear models but leave the dispersion parameter unrestricted and estimate it from the data. Unlike negative binomial models, the variance is assumed to be a linear function of the mean.This strategy leads to the same coefficient estimates as a standard Poisson model but standard errors are adjusted for over dispersion. Following the “dummy variable method” for fixed effects in Poisson models we included k – 1 dummy variables to represent the sample participants in each model. A series of models were fit sequentially to test the association of each one-year-lagged psychiatric problem domain with each substance use outcome. First, we regressed separately each one year-lagged shift in the average psychiatric problem T-scores on each substance use outcome. Within these models, age-related changes in substance use were controlled for using natural cubic splines. Natural cubic splines are a flexible smoothing approach for non-linear relationships, and are composed of piece wise polynomial functions that split the continuous age variable into separate line segments, each free to have its own shape. Segments are joined by “knots,” which we specified a priori to result in line segments for ages 13-14, 15-16, and 17-19. Slopes are constrained to converge at each knot . Second, we sequentially tested groups of potential confounders. All covariates were back-lagged two years, so that they would be modeled prior to the measurement of the exposure. This ensured that the estimated total effect of change in psychiatric problems on change in substance use included effects mediated through the covariates that occurred contemporaneous to changes in psychiatric problems. In our second set of models, we adjusted for age, SES, substance use variables that were not modeled as the outcome , and measures of psychiatric problems that were not the exposure of interest .