Summary Guidelines for good practice in cost-effectiveness analyses (CEAs) are available from the UK's National Institute for Health and Clinical Excellence, which address the scope of the CEA study, appropriateness of the data used and how to account for model as well as statistical uncertainty. Within these three broad headings, we identify 10 specific issues that may affect materially the CEA of a public health intervention for which injecting drug users are a major target group. The 10 issues relate to injecting drug users' under-representation in randomized controlled trials; their risk of blood-borne viruses—such as human immunodeficiency virus and hepatitis C virus—which have long-term chronic sequelae, their markedly higher age-specific mortality than in the general population and the relapsing–remitting nature of opiate dependence with its associated risk of overdose. We consider how adequately three key UK CEAs accounted for the relevant injecting drug user specific issues that we have highlighted. The three case-studies are antiviral treatment of carriage of hepatitic C virus, opiate substitution therapy and needle and syringe programmes.