AIMS: To characterise the clinical condition of rapid onset heroin overdose cases, and to determine the incidence and characteristics of such cases.

DESIGN: A retrospective cohort study of witnessed heroin overdose cases over a 12-month period (1 July 2023 to 30 June 2024).

SETTING: The Medically Supervised Injecting Room in Melbourne, Australia.

CASES: Individuals who experienced a rapid onset overdose. A control group of standard onset cases amongst those same individuals was used for comparison.

MEASUREMENTS: The severity of acute opioid toxicity for rapid versus standard onset heroin overdose cases, using a clinical severity scale regularly used at the Medically Supervised Injecting Room.

FINDINGS: From a total of 1727 heroin overdoses, 202 were identified as rapid onset cases that occurred amongst a cohort of 98 individuals. There were 997 standard overdoses from 75 individuals from this same cohort used as a control. Rapid onset overdoses presented with greater severity of respiratory depression. Forty-one percent (n = 82) of cases required positive pressure ventilation, including 19% (n = 38) classified as Grade 2 severity and 22% (n = 44) classified as Grade 3a severity. The standardised rate of rapid overdoses was approximately doubled on High overdose risk days (0.151/overdose) compared with that on Low overdose risk days (0.077/overdose).

CONCLUSION: Rapid onset heroin overdoses typically present as medical emergencies involving severe respiratory depression shortly after heroin administration. Some people appear more likely to experience a rapid onset overdose than others, but factors for this are unclear. Rapid onset heroin overdoses also more commonly occur on some days compared with others, which may indicate an interrelationship between personal risk and the unregulated drug market. Rapid onset overdose cases may play a substantial role in hypoxic-related morbidity and mortality associated with heroin overdoses.