Our past year estimate represents about 774,434 PWID in 2011 (range: 494,605–1,054,263). Tempalski and colleagues found a 2007 population estimate for PWID of approximately 1,500,000 with minimum and maximum estimates of approximately 1,300,000 and 1,700,000 million [10]. In view of the differences between our study and that by Tempalski and colleagues in terms of methods (meta-analysis vs. multiplier methods), datasets (national surveys vs. drug and HIV testing data), and time periods (2000–2008 vs. 1992–2007), and the lack of an accepted gold standard method for PWID population size estimates [6], it is unclear whether our estimate represents under-estimation or the Tempalski method represents over-estimation. The use of any PWID population size estimate should be accompanied by acknowledgement of the limitations of the methods and data sources.
