 This is a phase IIIII, randomized, double-blind, placebo-controlled study of the safety and efficacy of chemoprophylactic tenofovir, administered orally once daily to IDUs. The study will be conducted in Bangkok at 17 BMA Drug Treatment Clinics. Study participants will be randomized 1:1 to receive tenofovir 300 mg or placebo. Participants will be evaluated for adverse events and HIV seroconversion. 