may inject a few times throughout the day to forestall physical withdrawal. The cocaine user on a run may be more likely to share needles, due to the need for repeated administration of the drug within a short time in order to satisfy the intense craving to consume more of the drug. This illustrates the point that merely counting the number of injection drug users, defined in some fashion, may not be sufficient to adequately address the prevention needs of certain subgroups of injection drug users.

The National Research Council report found "the current estimates of the prevalence of IV drug use to be seriously flawed" (p. 233) and recommended that a high priority be given to research on the estimation of the number of injection drug users in the United States (Turner et al., 1989). Since that report, a special issue of the Journal of Drug Issues (Hser and Anglin, 1993) on prevalence estimation techniques for drug-using populations makes note of improvements in the quality and quantity of data available and convincingly documents a growing recognition of the importance of this area of inquiry. Acknowledging the imprecision of existing estimation models, Anglin et al. (1993) are more positive about the state of the field. By looking at prevalence estimation as policy research, they stress the potential of this work to inform ongoing decision making, thereby emphasizing appropriate but cautious application of estimates with less stress on absolute precision. In sum, although the continuing imprecision in estimates of the number of injection drug users remains a concern, this panel recognizes the need to make the best use of available data to inform the decisions of policy makers.

ESTIMATES OF INJECTION DRUG USERS

Despite the imprecision of available estimates in this country, most would agree that the magnitude of the population of drug injectors is a major influence on evolving patterns of HIV transmission. U.S. Public Health Service reports dating back to the 1980s all estimate the national figure for injection drug user to exceed 1 million. The Centers for Disease Control (1987a, 1987b) reported approximately 1.5 million injection drug users for 1986 and a revised figure of 1.1 million for 1987. One year later, the National Institute on Drug Abuse (NIDA) estimated that there were about 1.1 to 1.3 million injection drug users in the United States (Schuster, 1988). In its 1989 report, the National Research Council included a critique of the estimation methods used to generate these figures, suggesting that it was not unreasonable to believe that their margin of error could be as great as 100 percent (Spencer, 1989). The report cautions, "That is to say, the true number of IV drug users could be as few as half a million or as great as 2 million" (Turner et al., 1989:230).

Scott Holmberg (1993, 1994) of the Centers for Disease Control and



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