Sample

Our sample is based on repeated cross-sectional data from the 1985, 1988, and 1990 National Survey on Drug Use and Health (NSDUH) for the ADAA cohort and the 2009, 2012, and 2013 surveys for the FSA cohort [32]. NSDUH, previously known as the National Household Survey on Drug Abuse, is a nationally-representative cross-sectional household survey sponsored by the Substance Abuse and Mental Health Services Administration (SAMHSA) [33]. Using a multi-stage area probability design, the survey gathers detailed information on tobacco, alcohol, and illicit drug use from households and individuals. The NSDUH pioneered several methodological innovations to enhance validity of the data, and reliability studies have shown consistent responses for lifetime and past year drug use [34]. In addition to NSDUH-specific validity reports, self-reported illicit drug use, in general, is considered a valid and reliable means of measurement [35, 36]. Response rates for these five years ranged from 77% in 1988 to 88% in 2009. The NSDUH is publicly available from SAMHSA and all participants provided informed consent.

We exclude children under the age of 15, since they received only a partial survey, as well as observations with missing information on cocaine base or powder cocaine use or a key covariate. We further exclude respondents who report having misused a prescription opioid (defined as having used an opiate for non-medical purposes, i.e. “to get high”) or used heroin in any form in the past year from both ADAA and FSA analyses in order to avoid potential confounding factors associated with the rising opioid epidemic. The final analytic sample size is 21,296 for the ADAA cohort and 130,574 for the FSA cohort reflecting the increase in the overall NSDUH sample between these time periods.

Outcome measures

We define cocaine base as any freebase or crack cocaine use within the previous 12 months. The 1985 NSDUH survey was the first year that cocaine base was included in the questionnaire. In this survey, respondents were asked about any type of cocaine use in the past 12 months, and lifetime use of freebase cocaine. If the respondent reported having used any type of cocaine within the last year and freebase at any point, we categorized them as having used cocaine base within the last year. Although this may lead to overestimation of 1985 cocaine base use, this overestimation is likely minimal since this form of the drug did not emerge outside of select regions until roughly 1984 and is considered to have been widespread only beginning in 1985 [14, 37]. However, if there were an overestimation of cocaine base use in 1985, this would exaggerate our estimates in change of use of cocaine base following the ADAA, but should not affect the estimates for powder cocaine. For later surveys, respondents were directly asked about crack or freebase cocaine use within the last 12 months.

While crack and freebase cocaine are not identical forms of cocaine, both are considered cocaine base under the ADAA and FSA laws. For our ADAA models we use freebase use as our cocaine base indicator, since this terminology was most consistently understood during this period. While not all freebase users during this period reported crack use, all crack users reported freebase use. Over the FSA period, freebase use was no longer included in the survey since terminology had changed so that “crack” was the more recognizable term. Since definitions are consistent within time periods and, therefore, across grouped models, no biases should be introduced by the change in terminology.

Powder cocaine includes only those who have used powder cocaine within the past 12 months, but have not used cocaine base. If a person uses both forms of cocaine, they are categorized as cocaine base users; we conducted sensitivity analyses to assess the impact of this classification strategy (described in Sensitivity Analyses section).

Prescription drug misuse in the past 12 months is defined as use of prescription sedatives and/or tranquilizers (e.g., Quaalude, Xanax, or Valium) either without a prescription, in greater amounts than prescribed, more often than prescribed, or for any non-medical reason (i.e., to get high). Sedative and tranquilizer use was common in the 1960s and 1970s, and these medications were among the most commonly-prescribed psychoactive drugs in the US; both are associated with physiologic dependence and are considered potential drugs of abuse [38]. These drugs were selected because although the ADAA focused on cocaine base, it covered all commonly-used illicit drugs at the time, including marijuana, and no other campaigns contemporaneously targeted misuse of these substances. Narcotics or opiates are not included in our categorization of prescription medications of abuse due to their cyclic popularity and the potential confounding effects of the recent opioid epidemic. In fact, we exclude respondents who report past-year opioid use to ensure that confounders are not introduced. Opiates are further addressed through additional models described in the Sensitivity Analyses section below. The medications included in the prescription medication negative control were consistently assessed in all six survey years. By using prescription drugs as a negative control, we are able to isolate changes in cocaine use due to sentencing policies from temporal trends in drug use more generally.

Study design

We model the ADAA and FSA separately, first to address differential implementation of mandatory minimum sentences by cocaine type in the ADAA, and then extend this analysis using the FSA to assess changes in drug use following relaxation of this sentence discrepancy. Three models per law are analyzed. The first model estimates the change in cocaine base use after law implementation. The second model estimates the change in powder cocaine use. If mandatory minimums drive the change in drug use, we would expect that use would decrease for both forms of cocaine, but that this decrease should be greater for cocaine base due to harsher penalties. Finally, to ensure that any change in drug use is in fact a result of sentencing guidelines, we fit a third model as a negative control: prescription drug misuse, indexed as misuse of tranquilizers and/or sedatives in the past year. Misuse of prescription medications was not targeted in either the ADAA or FSA laws, and thus should not have been affected by changes to sentencing guidelines for other drugs [36].

We use three years of survey data for both the ADAA and FSA models. As the ADAA was enacted in 1986, 1985 is considered the baseline year. Since drug use is defined as past year use, 1988 is used as the post-implementation year to allow for a full year of drug use after ADAA enactment. However, due to an amendment to the ADAA pertaining to possession sentencing in 1988, 1990 is also included to account for full implementation of the law (further described in the Sensitivity Analyses section). Similarly, the Fair Sentencing Act was enacted in August of 2010. Therefore, in order to obtain full-year drug use estimates after implementation, we use the 2012 NSDUH survey response. We further add 2013 responses to ensure robust estimation of post-FSA changes in drug use. As both the ADAA and the FSA were popular, bipartisan bills passed in election years, both laws were widely publicized as part of congressional campaigns and enacted immediately; therefore, two post-implementation years apiece should be sufficient to estimate changes in behavior related to sentencing reforms. We do conduct a sensitivity analysis around this time frame (as described in the Sensitivity Analyses section) and results are robust to additional years.

Statistical analysis

We estimate the change in drug use using three identical multivariable logistic models per law, one for each drug. Drug use is defined as a binary indicator of having used within the past 12 months or not. All regressions are weighted to account for complex survey design in the standard errors. Using the principles described by Paternoster et al., a Z-test is used to compare coefficients across the models to determine if there is a differential impact of the law on cocaine base versus powder cocaine versus prescription drug misuse [39]. We use dummy variables for years to identify the likelihood of reporting drug use in the post-implementation period compared to the pre-implementation period. All models are adjusted for age (categorized as older or younger than 25 years to accommodate the oversampling of younger age groups), race (categorized as non-Hispanic white, non-Hispanic black, Hispanic, or other), gender, marital status (categorized as married, never married, and separated/widowed/divorced), education level (categorized as under 18, less than high school, high school graduate, some college, and college graduate), and income level (categorized by modified income quartiles). Models are also adjusted for past year use of licit (alcohol and tobacco) and other illicit (marijuana) drugs.

We use the Akaike information criterion for model selection and verified goodness of fit with a survey weight-adjusted Hosmer-Lemeshow test and Pregibon linktest. Since all three models had to be identical to allow for comparison, the three models had varying levels of fit. However, unadjusted simple logistic regressions of outcomes all indicated adequate specification. These estimates were nearly identical to adjusted regression estimates (available upon request).

Analyses were conducted with StataSE 14 statistical software using survey procedures to account for the study design. All reported p-values refer to two-tailed tests.

Sensitivity analyses

Categorization of cocaine use: We re-categorized respondents who used both cocaine base and powder cocaine as powder cocaine users (N = 1489 in the ADAA cohort and N = 2847 in the FSA cohort), and find our results to be consistent regardless of categorization.

Modification of the ADAA: In 1988, the U.S. Congress modified the ADAA to include drug possession in sentencing guidelines in addition to drug trafficking. In our main analysis, we use 1990 as the post-implementation year to ensure we are capturing full implementation. However, we estimate the change in drug use in 1988 as well to monitor any variation in use that might be present as the law is expanded and revised. We also conduct an analysis including 1991 in the post-implementation period, which again provides similar estimates to the main analysis.

Opiate users: To account for possible substitution effects of users switching from one drug, whether cocaine or other prescription medications, to opioids, we exclude respondents with non-medical use of prescription opioids or who used heroin in any form in the past year from both ADAA and FSA analyses. However, to ensure that these respondents are not fundamentally different, due to potential differences in level of addiction or dependence or risk seeking behavior, we perform a sensitivity analysis including these respondents. While individual model estimates varied somewhat from primary model estimates, overall conclusions remained consistent.