Prevention has recently attempted to estimate the number of "current active" injection drug users in the 96 largest metropolitan statistical areas (MSAs) in the United States—those with more than 500,000 inhabitants. Holmberg's estimated number of injection drug users in these 96 MSAs is approximately 1.5 million, which is in line with other estimates. Although the 96 MSAs have only about 160 million persons, or about 62 percent of the entire U.S. population of 260 million, it is likely that they include a much higher percentage of all injection drug users. Holmberg (1993, 1994) notes that these areas include about 85 percent of all reported AIDS cases, and it is likely that they include approximately the same proportion of injection drug users.1 Thus, if at least 85 percent of all injection drug users are located in the 96 MSAs, that suggests a national total of injection drug users on the order of 1.7 million. Holmberg's detailed procedure for estimating the number of injection drug users in these metropolitan statistical areas consisted of compiling available information from diverse sources (i.e., published and unpublished). Specific studies of injection drug users performed by federal agencies, health departments, drug treatment services, academic institutions, and a large government contractor (i.e., Research Triangle Institute) were reviewed to arrive at estimates for individual MSAs. Holmberg's procedure included several criteria to ascertain the reasonableness of his estimates: (1) the estimated number of injection drug users should be consistent with the proportion of known injection drug users in treatment (reflecting NIDA estimates of the percentage of injection drug users in treatment); (2) the number should likewise correspond to the proportion of known injection drug users tested for HIV at confidential counseling and testing sites, where such information was available; (3) the estimated proportions of injection drug users should be consistent with the stratification of high-, medium-, and low-injection-use cities, as defined by the Drug Abuse Warning Network (DAWN) data, treatment providers, and ethnographers.

Geographic Variations

With respect to regional and geographic variation, the pattern is one of clusters of injection drug users, and the clusters are well dispersed across the country. Half of all injection drug users (in the 96 largest MSAs) are found in the top 16 MSAs. Of the top 11 MSAs, 6 are part of the Northeast corridor: Boston, New York City, Newark, Philadelphia, Baltimore, and Washington, D.C. Nearly a quarter (23.5 percent) of all injection drug users (in the 96 largest MSAs) are located in just these 6 MSAs. The other three-quarters are located in the other 10 MSAs: Los Angeles, Chicago, Houston, Detroit, Miami, Riverside, San Francisco, Atlanta, San Juan, and Fort Worth. Three of these are on the West Coast, two in the upper Mid-west,



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