This article examines the human rights and public health implications of injection drug use in prisons with a specific focus on HIV and hepatitis C (HCV) viruses. The authors argue that prisoners who inject drugs have a right to access harm reduction measures--those that reduce the harmful consequences of drug use without necessarily reducing drug consumption. Moreover, states that fulfill their obligation to provide prisoners with harm reduction measures such as access to bleach, substitution therapy, and sterile injection equipment implement sound public health policy with a positive impact for a population particularly vulnerable to HIV and HCV. Ultimately, this approach benefits not only prisoners but also prison staff and the public, and does not entail lessening of the safety and security of prisons.