Are prescription misuse and illicit drug use etiologically distinct? A genetically-informed analysis of opioids and stimulants

Specificity in genetic and environmental risk for prescription opioid misuse and heroin use

BACKGROUND

Many studies aggregate prescription opioid misuse (POM) and heroin use into a single phenotype, but emerging evidence suggests that their genetic and environmental influences may be partially distinct.

METHODS

In total, 7164 individual twins (84.12% complete pairs; 59.81% female; mean age = 30.58 years) from the Australian Twin Registry reported their lifetime misuse of prescription opioids, stimulants, and sedatives, and lifetime use of heroin, cannabis, cocaine/crack, illicit stimulants, hallucinogens, inhalants, solvents, and dissociatives via telephone interview. Independent pathway models (IPMs) and common pathway models (CPMs) partitioned the variance of drug use phenotypes into general and drug-specific genetic (a), common environmental (c), and unique environmental factors (e).

RESULTS

An IPM with one general a and one general e factor and a one-factor CPM provided comparable fit to the data. General factors accounted for 55% (a = 14%, e = 41%) and 79% (a = 64%, e = 15%) of the respective variation in POM and heroin use in the IPM, and 25% (a = 12%, c = 8%, e = 5%) and 80% (a = 38%, c = 27%, e = 15%) of the respective variation in POM and heroin use in the CPM. Across both models, POM emerged with substantial drug-specific genetic influence (26-39% of total phenotypic variance; 69-74% of genetic variance); heroin use did not (0% of total phenotypic variance; 0% of genetic variance in both models). Prescription sedative misuse also demonstrated significant drug-specific genetic variance.

CONCLUSIONS

Genetic variation in POM, but not heroin use, is predominantly drug-specific. Misuse of prescription medications that reduce experiences of subjective distress may be partially influenced by sources of genetic variation separate from illicit drug use.

Contextualizing prescription opioid misuse and heroin use within dimensional models of drug involvement

Background

Prescription opioid misuse (POM) is often implicated in heroin initiation, despite evidence that POM does not predict heroin initiation any better than other drug use. Additionally, prescription misuse and illicit use behaviors tend to respectively "cluster" together. This study aimed to test a series of theory-driven factor models to explore how POM and heroin use are situated within the broader constellation of drug use that typically occurs alongside opioid (mis)use.

Methods

36,309 individuals from NESARC-III (56.31% female; mean age=45.63 [SD=17.53]) reported their lifetime (mis)use of prescription opioids, prescription stimulants, prescription sedatives, heroin, cannabis, cocaine/crack, illicit stimulants (e.g., methamphetamine), club drugs, hallucinogens, and inhalants, and were administered a DSM-5 substance use disorder (SUD) assessment. Bifactor, correlated factors, and one-factor confirmatory factor models were fit using all drug use/SUD variables and subsequently compared.

Results

POM was most strongly correlated with prescription sedative misuse; heroin use was most strongly correlated with cocaine/crack use. All factor models fit the data well. Highly correlated factors and patterns of factor loadings suggested that POM and heroin use were most parsimoniously captured within a general factor alongside all other forms of drug use. This was also the case for SUD. Additional analyses testing an alternate factor structure provided further support for unidimensionality.

Conclusions

POM and heroin use, as well as prescription- and heroin-based SUDs, were neither separable nor distinctly associated. Future research should account for other drug use more comprehensively rather than isolating POM as a primary risk factor in heroin use and use disorder.