"In summary, data from studies conducted in Australia, Europe, Asia and the United States have, with few exceptions, found strong associations between participation in methadone treatment and reductions in the frequency of opioid use, fewer injections and injection-related HIV risk behaviors, and lower rates of HIV prevalence and incidence. Few randomized controlled trials have been conducted due to ethical concerns regarding the random assignment of individuals to no treatment or other potentially less effective treatment modalities. Despite this fact, the consistency of findings from the observational and case-controlled studies cited here provide a preponderance of evidence suggesting that sustained treatment of opioid-dependent injection drug users with methadone is associated strongly with protection from HIV infection."
Sullivan, Lynn David S. Metzger, Paul J. Fudala & David A. Fiellin, "Decreasing International HIV Transmission: The Role of Expanding Access to Opioid Agonist Therapies for Injection Drug Users," Addiction, February 2005, Vol. 100, No. 2, p. 152.