The current study found a direct significant relationship between current age and age of first heroin use and first heroin injection. This result is consistent with other Australian research [17,18]. Nevertheless the finding is subject to bias due to 'right censoring' of the data [17]. For example, an 18 year old recruited into the study cannot have an age of initiation above 18 years, while a 30 year old can have any age of initiation up to 30 years, and be included in the study even if they commenced injecting at 25 years [for a more detailed discussion see [17]]. There are numerous explanations for this decrease in initiation to heroin use; the rapid expansion of the Australian heroin market between 1996 and 2000, where the price of heroin decreased concomitant with an increase in purity and availability from 1996 to 2000 [32], may in part explain this phenomenon, though greater examination of the structural determinants of drug use are also warranted [33].
