Trends and psychosocial correlates of same day polysubstance use among people who inject drugs in Australia, 2012-2022

•Same day polysubstance use was common among this sample of people who inject drugs.•Use of opioids and other depressants on the same day decreased over the study period.•There was no increase in same day use of stimulants with opioids and/or other depressants.•Polysubstance use was associated with psychosocial factors, such as unstable housing.•Polysubstance use was also associated with multiple drug use risk behaviours.