Approximately 10% of new HIV infections worldwide are attributable to injecting drug use, often of an opiate such as heroin.1 Opioid substitution therapy supplies illicit drug users with a replacement drug, a prescribed medicine such as methadone or buprenorphine, which is usually administered orally in a supervised clinical setting. The effectiveness of this therapy is recognized in developed countries, where the provision of opioid substitutes to opiate-dependent people is a fundamental component of the response to the dual public health problems of injecting drug use and HIV transmission.2 However, better prevention of HIV transmission among and from injecting drug users is still needed, especially in resource-poor settings.