Full length articleNonmedical prescription opioids and pathways of drug involvement in the US: Generational differences
Introduction
Heroin use increased sharply over the last several years among nonmedical users of prescription opioids (NMPO), especially heavy and dependent users (Banerjee et al., 2016, Cerdá et al., 2015, Jones, 2013, Jones et al., 2015, Martins et al., 2016, Muhuri et al., 2013). NMPO users are also more likely than non-users to use marijuana, stimulants or cocaine (Banerjee et al., 2016, Boyd et al., 2009, Catalano et al., 2011, Han et al., 2015, McCabe et al., 2011, Wu et al., 2008, Young et al., 2012). The prevalence of legal and illegal drug use, including NMPO, has been examined across birth cohorts (Bluthenthal et al., 2017, Degenhardt et al., 2007, Golub and Johnson, 2001, Johnson and Gerstein, 1998). Use of heroin and marijuana peaked in the early 1970’s, cocaine in the 1980’s, and NMPO in the mid-2000’s. Progression through stages of drug use from nonuse to alcohol/tobacco, marijuana, and hard drugs (cocaine, heroin) observed from 1979 to 1997 for 1910–1971 birth cohorts was examined by Golub and Johnson (2001). Progression to each stage peaked with the 1960 birth cohort. To the best of our knowledge, except for that study, generational changes in developmental patterns of drug initiation in the population, in particular, pathways involving NMPO and NMPO in relation to cocaine and heroin, have not been examined.
Drug usage starts with alcohol or cigarettes and proceeds to illegal drugs, even in recent periods when rates of marijuana use surpass those of cigarette use (Keyes et al., 2016). Marijuana, in turn, precedes use of cocaine and other illicit substances (Cleveland and Wiebe, 2008, Degenhardt et al., 2010, Fergusson et al., 2006, Kandel, 2002, Kandel et al., 2006, Lynskey et al., 2012, Rebellon and Van Gundy, 2006, Wagner and Anthony, 2002). This progression, observed in the US and internationally, led to the notion of the Gateway Hypothesis: alcohol, tobacco or marijuana are gateways to using other substances. The notion of developmental stages in drug behavior does not imply that these stages are obligatory nor that entry into a lower stage drug inexorably leads to higher stage drugs. Translational research in rodents supports a causal mechanism through which the gateway sequence arises between two drugs. Nicotine pretreatment (in mice) and alcohol (in rats) enhances responses to later cocaine exposure but not vice versa (Griffin et al., 2017, Kandel and Kandel, 2014, Levine et al., 2011). Nicotine acts as a gateway drug and exerts a priming effect on cocaine through increased global histone acetylation in the nucleus accumbens, and this creates an environment primed for induction of gene expression.
Another perspective, the Common Liability Model, posits that use of multiple drugs reflects a common liability for drug use, with no specific influence of one particular drug leading to use of another (Palmer et al., 2009, Vanyukov et al., 2012). Generalized risks include not only common genetic predispositions but psychosocial and environmental factors including drug availability (Bailey et al., 2011, Cleveland and Wiebe, 2008, Rebellon and Van Gundy, 2006, Wagner and Anthony, 2002).
The position of NMPO use in the sequence of drug involvement remains to be established, especially nationally, and may vary between birth cohorts who differ in their drug experiences. Catalano et al. (2011) inferred an order between NMPO and other drugs from rates of use of different drugs from first grade to age 21. While Harrell and Broman (2009) concluded that alcohol and marijuana use in adolescence predicted any nonmedical prescription drug (NMPD) use six years later, including NMPO, the sequence of initiation among these drugs could not be identified because neither NMPD use at the initial interview nor onset age were ascertained. Others reported that marijuana use prior to age 18 was associated with NMPO use by age 25 (Fiellin et al., 2013), and cigarette and marijuana use by 12th grade predicted NMPO use by age 23 (Miech et al., 2015). Several studies documented that initiation of NMPO use occurred before heroin (Banerjee et al., 2016, Cerdá et al., 2015, Jones, 2013, Muhuri et al., 2013, Novak et al., 2016). This sequence became more prevalent in the population between 2002–04 and 2008–10 (Jones, 2013) and among those born after 1980 (Novak et al., 2016). Except for these studies, changes in patterns of heroin initiation in relation to NMPO and other drugs in different historical periods and at different points in the lifespan have not been examined.
Drug use is an age-graded behavior, and historical differences in prevalence of drug use experienced by different birth cohorts at ages at highest risk for drug initiation may influence the drug use careers of different generations. We consider developmental patterns of use across three generational cohorts spanning ages 18–64 in 2013–2014: Millennials (born in 1979–96), Generation X (born in 1964–79), and Baby Boomers (born in 1949–64). The birth years defining generations vary slightly across investigations (Fry, 2016, Pew Research Center, 2015), but generations provide a useful way of considering the behaviors of individuals born in different time periods. Trend data as of 1982 among 18–34 years olds illustrate that, while prevalence of lifetime use of different drug classes varied greatly over the last 40 years, the relative ranking of these drugs remained the same, except for NMPO (Supplementary Fig. 1). At ages 18–34, Baby Boomers lived through a period of increased drug experimentation, while Generation X lived through a period of decreasing prevalence of use of different drugs. Millennials experienced increases in use, especially for NMPO and heroin. Hence, we would expect sequences of drug initiation to vary across generations, especially regarding the position of NMPO.
We analyze developmental patterns of involvement in legal and illegal drugs with a focus on sequences between NMPO, cocaine, and heroin. Using a novel simulation method that distinguishes significant drug initiation sequences from patterns expected to occur by chance due to correlations induced by common liability between use of different drugs, we address the following questions: (1) what is the position of NMPO use in drug initiation sequences in three generations (Millennials, Generation X, and Baby Boomers?) and (2) what are gender and racial/ethnic differences in these patterns among Millennials, the youngest generation?
Section snippets
Sample
Data are from two aggregated surveys (2013–2014) from the National Survey on Drug Use and Health (NSDUH), annual cross-sectional surveys of drug use in multistage representative probability samples of the US population aged 12 and older (CBHSQ, 2015a, CBHSQ, 2015b). All states are represented. The target civilian non-instituionalized population represents over 98% of the population. Persons in non-institutional group quarters (homeless shelters, rooming houses, college dormitories) and
Lifetime use of five drug classes among generations
While the absolute prevalence of lifetime use of the drugs varied across generations, relative rankings were similar except for NMPO and cocaine (Table 1). Alcohol or cigarette use was most prevalent. Marijuana use was next in prevalence, with rates higher among Millennials and Baby Boomers than Generation X. NMPO use increased among each successively younger generation while cocaine use decreased. Among Millennials, NMPO use was more prevalent [21.5% (95% CI = 20.9%–22.1%) than cocaine use
Discussion
Across three generations in the US, NMPO use fits within a developmental sequence of drug involvement that starts with alcohol or cigarettes, proceeds to marijuana, then to NMPO and/or cocaine, and finally to heroin. The combined rates of initiating either NMPO or cocaine among marijuana users do not vary across generations, whereas the sequential patterns between the two drugs do. Among Millennials, NMPO is more likely than cocaine to follow directly after marijuana, and the reverse occurs
Role of funding
This research was supported by grant R01 DA036748 from the National Institute on Drug Abuse (D. Kandel, PI). Support was also provided by the New York State Psychiatric Institute (P. Griesler). The funding agency had no part in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; or decision to submit the manuscript for publication.
Contributors
All the authors contributed to the design of the analysis, reviewed the analysis, and participated in the writing of the manuscript. All authors approved of the final version of the manuscript before submission.
Conflict of interest disclosures
The authors report no conflicts of interest.
Acknowledgements
We thank Mr. Benjamin Jenkins for his assistance in the preparation of the manuscript. We would also like to thank the reviewers of the manuscript for their constructive critiques and suggestions.
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