Abstract
Background Unlike many other psychiatric conditions, individuals with drug dependence pass through a number of intermediate stages before developing a clinical condition. Studying variation in speeds of transitions between these stages provides novel insights into Substance Use Disorder aetiology.Aims To explore:
1. Relationships between early cannabis transitions and later outcomes;
2. Relationships between individual, childhood, mental health and drug use factors and transitions;
3. The extent to which speed of early cannabis transitions are influenced by genes and environment, and the genetic correlation with dependence;
4. The relationship between early heroin transitions and later heroin outcomes, and the effect of route of administration.
Design Three samples were analysed: 1) 3824 Australian twins and siblings; 2) 93 opiate substitution treatment clients; 3) 408 heroin users.
Methods Cox PH Survival Analysis; Regression Analysis; Classic and Bivariate Twin Modelling.
Findings Early onset of first opportunity to use (OTU) cannabis was associated with increased risks of later cannabis use outcomes, with those who reported first OTU before age 14 being twice as likely to report cannabis daily use, abuse/dependence or treatment-seeking relative to those whose first OTU occurred after age 18.
Faster progression to subsequent cannabis use was associated with increased risks of later cannabis use outcomes, with those whose subsequent use was within a week of initiation being twice as likely to report cannabis daily use, and more likely to abuse/dependence, relative to those whose subsequent use was more than year after initiation.
Conduct disorder, weekly tobacco use, male gender and parental drug problems were associated with faster progression to both OTU cannabis, and from opportunity to dependence. A genetic correlation of .54 was observed between age of OTU cannabis and cannabis abuse/dependence. Early OTU heroin was associated with three times likelihood of overdose and neck/groin injecting. Injecting heroin was associated with faster progression to daily use.
Conclusions Considering the stage-sequential nature of drug use has identified factors associated with dependence that also influence behaviour from the earliest stage of drug involvement, and has revealed potential targets for intervention.
| Date of Award | 2017 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | John Strang (Supervisor), Michael Lynskey (Supervisor) & Katherine Morley (Supervisor) |