Introduction: We examine how smoking and injecting methamphetamine change over time and correlate with specific health and social outcomes.

Methods: Panel data from a longitudinal cohort dependent on methamphetamine (N = 444; 891 months). Random effects regression models examined the relationship between smoking and/or injecting methamphetamine and past month outcomes (substance use, ≥daily injection, needle/syringe sharing, psychological distress, poor mental and physical health, sexual behaviour, psychotic symptoms, violent behaviour and crime). Effects were adjusted for between-group differences at baseline.

Results: At baseline, 56% of participants only injected methamphetamine in the past month, 18% only smoked and 26% both injected and smoked (concurrent injecting and smoking). Compared to injecting only, concurrent injecting and smoking was associated with more days of methamphetamine use (b = 1.3, P < 0.001; adjusted [A] b = 1.2, P < 0.001), more frequent injection [odds ratio (OR) 1.8, P = 0.013; adjusted OR (AOR) 1.6, P = 0.042], violent behaviour (OR 2.1, P = 0.001; AOR 1.8, P = 0.013] and crime (OR 3.1, P < 0.001; AOR 2.5, P < 0.001). Non-injecting related outcomes did not differ significantly for only smoking versus only injecting. There was no significant transition from injecting methamphetamine at baseline to non-injecting methamphetamine use at follow up, or from exclusively smoking methamphetamine at baseline to any methamphetamine injection at follow up.

Discussion and conclusion: Efforts are needed to address heavier methamphetamine use, more frequent drug injection and elevated violent behaviour and crime among people who concurrently smoke and inject methamphetamine.