Aims Previous studies have largely attributed the Australian heroin shortage to increases in local law enforcement efforts. Because western Canada receives heroin from similar source nations, but has not measurably increased enforcement practices or funding levels, we sought to examine trends in Canadian heroin-related indices before and after the Australian heroin shortage, which began in approximately January 2001. Methods During periods before and after January 2001, we examined the number of fatal overdoses and ambulance responses to heroin-related overdoses that required the use of naloxone in British Columbia, Canada. As an overall marker of Canadian supply reduction, we also examined the quantity of heroin seized during this period. Lastly, we examined trends in daily heroin use among injection drug users enrolled in the Vancouver Injection Drug Users Study (VIDUS). Results There was a 35% reduction in overdose deaths, from an annual average of 297 deaths during the years 1998–2000 in comparison to an average of 192 deaths during 2001–03. Similarly, use of naloxone declined 45% in the period coinciding with the Australian heroin shortage. Interestingly, the weight of Canadian heroin seized declined 64% coinciding with the Australian heroin shortage, from an average of 184 kg during 1998–2000 to 67 kg on average during 2001–03. Among 1587 VIDUS participants, the period coinciding with the Australian heroin shortage was associated independently with reduced daily injection of heroin [adjusted odds ratio: 0.55 (95% Cl: 0.50–0.61); P < 0.001]. Conclusions Massive decreases in three independent markers of heroin use have been observed in western Canada coinciding with the Australian heroin shortage, despite no increases in funding to Canadian enforcement efforts. Markedly reduced Canadian seizure activity also coincided with the Australian heroin shortage. These findings suggest that external global heroin supply forces deserve greater investigation and credence as a potential explanation for the Australian heroin shortage.