Changes in injection risk behavior associated with participation in the Seattle needle-exchange program.

We studied the influence of the Seattle, Washington, needle-exchange program on sharing of drug injection equipment (syringes, drug cookers, filtration cotton) to identify potential gaps in risk reduction and to understand in greater detail the lack of an association between exchange use and risk of hepatitis B or C virus transmission. In a cohort of 2,208 injection drug users who completed a 1-year follow-up visit, we measured the association between needle-exchange use at study enrollment (ever vs. never) and injection risk behavior at the follow-up. Control for confounding was carried out using both logistic regression and propensity score analytic methods to estimate the adjusted odds ratio (AOR). In both univariate and multivariate analyses, needle-exchange use was associated with a lower likelihood of injection with a used syringe (AOR = 0.7, 95% confidence limit 0.5, 0.9). There was no association between exchange use and cooker or cotton sharing (AOR = 0.8, 95% confidence limit 0.6, 1.1) or between exchange use and use of a common syringe to divide drugs (AOR = 0.9). This analysis suggests that risk reduction measures adopted by users of the Seattle exchange may not be sufficient to prevent transmission of all blood-borne viruses, including hepatitis C virus. Greater awareness of the infection risk associated with these practices may help curb this type of equipment sharing and ultimately prevent disease transmission.