We characterize the demographics, injection practices and risk behaviours of 1,158 injection drug users (IDUs) in Chennai, the capital of Tamil Nadu in southern India, who were recruited during 2005-2006 by community outreach. The median age was 35 years; the majority of IDUs were male, of Tamil ethnicity and married, and earning less than US$75 per month. Most (76%) had injected in the prior month. The median age at first injection was 25 years; the most common drug injected was heroin (80%) followed by buprenorphine. High risk behaviours were common and included needle-sharing, unsafe disposal, and inappropriate cleaning of needles as well as limited condom use. IDUs in India need to be educated on harm reduction and safe-injection practices; Pharmacies could serve as potential venues for HIV prevention interventions among IDUs in India, as most IDUs obtain their needles from pharmacies without prescription.