Despite evidence that the majority of injection drug users (IDU) are polysubstance users and tend to have extensive substance use histories at time of first injection, little is known about the patterns of substance use associated with injection initiation.

To determine how injection drug use was associated with specific patterns of multiple substance use. Both concurrent (any use during a defined time period) and simultaneous (same session) polysubstance use patterns were assessed.

IDU receiving treatment at a low-threshold methadone maintenance clinic in Halifax, Nova Scotia, Canada completed structured interviews regarding their substance use histories.

Injection coincided with: concurrent heavy alcohol use; increasing use of stimulants such as cocaine; and the introduction of misused prescription medications. During their most recent injection session, participants were significantly less likely to inject a stimulant, and were significantly more likely to identify an opioid as their primary drug of injection, when compared to injection initiation. Moreover, alcohol was more likely to be co-administered at time of first-ever injection, whereas prescription sedatives and crack cocaine were more likely to be co-administered during the most recent injection.

Initiation into injection tended to occur after an extensive and relatively consistent sequence of substance use. First ever and most recent injection drug use often occurred within a polysubstance context; however, the specific patterns of substance use changed with time. Findings suggest that specific patterns of multiple substance use may be appropriate targets for treatment and prevention of injection drug use.