Identification of Distinct Etiologies across Prescription Misuse and Illicit Drug Use: Opioids and Stimulants Compared
Background. Drug classes are grouped based on their chemical and pharmacological properties, but prescription and illicit drugs differ in other important ways. Opioid and stimulant classes contain prescription and illicit forms differentially associated with salient risk factors (common route of administration, legality), making them useful comparators for examining the potential differences in the etiological influences on (mis)use of prescription and illicit drugs. Methods. 2,410 individual Australian twins (Mage=31.77 [SD=2.48]; 67% women) were interviewed about prescription misuse and illicit use of opioids and stimulants. Univariate and bivariate biometric models partitioned variances and covariances into additive genetic, shared environmental, and unique environmental influences across drug types. Results. Variation in the propensity to misuse prescription opioids was primarily attributable to genes (37%) and unique environment (59%). Illicit opioid use was attributable to shared (71%) and unique (29%) environment. Prescription stimulant misuse was primarily attributable to genes (78%) and unique environment (21%). Illicit stimulant use was influenced by genes (48%), and shared (29%) and unique environment (23%). There was evidence for genetic influence common to both stimulant types, but limited evidence for genetic influence common to both opioid types. Conclusions. Prescription opioid misuse may share little genetic influence with illicit opioid use. Future research may consider avoiding unitary drug classifications, particularly when examining genetic influences.