Background and aims: There are ongoing concerns regarding the extra-medical use of quetiapine and related harms. We aimed to investigate trends in quetiapine use and extra-medical use among people who regularly inject drugs (PWID) in Australia between 2011-2018, and examine changes in the characteristics associated with extra-medical use of quetiapine, comparing those reporting use in 2011 and 2018.

Methods: This study examines eight years of survey data (2011-2018; n = 7,135 PWID) from Australia's Illicit Drug Reporting System. Linear regression was used to analyse trends over time, and multinomial logistic regression used to identify factors associated with extra-medical quetiapine use in 2011 and 2018.

Findings: The percentage of PWID reporting extra-medical quetiapine use decreased from 14.9 % in 2011 to 12.0 % in 2018; ranging between 10.5 %-15.8 % across years, and reported use was typically infrequent (less than once a month). In both 2011 and 2018, extra-medical quetiapine use was associated with use of benzodiazepines (2011 Adjusted Odds Ratio (AOR) 4.76, 95 % confidence interval (Cl) 2.31-9.82; 2018 AOR 3.10, 95 % Cl 1.84-5.23) and stimulants (2011 AOR 2.81, 95 % Cl 1.51-5.21; 2018 AOR 2.68, 95 % Cl 1.32-5.46) in the past six months.

Conclusions: Slightly more than one in ten PWID reported extra-medical quetiapine use between 2011-2018. Overall, the frequency of extra-medical quetiapine use among PWID was low, however, individuals often engaged in polysubstance use that has the potential to cause other drug-related harms. Targeted harm-reduction interventions focused on polysubstance use are therefore warranted to reduce potential risks among this group.