"While it is not feasible to do a randomized controlled trial of the effectiveness of needle or syringe exchange programs (NEPs/SEPs) in reducing HIV incidence, the majority of studies have shown that NEPs/SEPs are strongly associated with reductions in the spread of HIV when used as a component of comprehensive approach to HIV prevention. NEPs/SEPs increase the availability of sterile syringes and other injection equipment, and for exchange participants, this decreases the fraction of needles in circulation that are contaminated. This lower fraction of contaminated needles reduces the risk of injection with a contaminated needle and lowers the risk of HIV transmission.
"In addition to decreasing HIV infected needles in circulation through the physical exchange of syringes, most NEPs/SEPs are part of a comprehensive HIV prevention effort that may include education on risk reduction, and referral to drug addiction treatment, job or other social services, and these interventions may be responsible for a significant part of the overall effectiveness of NEPs/SEPs. NEPs/SEPs also provide an opportunity to reach out to populations that are often difficult to engage in treatment."
Nora Volkow, Director, US National Institute on Drug Abuse, Correspondence with Allan Clear, "NIH Response on Harm Reduction and Needle Exchange," Aug. 4, 2004.