This paper explores the varied and potentially conflicting goals of preventive measures as applied to opiate use. These goals relate both to the overall extent of opiate use and the amount of harm arising from this drug use. Prevention involves the control both of the supply side and the demand for opiates. Particular targets may include sharing equipment, injecting, progression to more harmful routes of administration, prevention of development of dependence, harmful sequelae of opiate use and co-morbidity. There may be a place for manipulating the black market for opiates to channel users in directions that achieve particular prevention goals, and being more selective about targeting particular groups of users at particular times. It is the role of scientists to quantify the dimensions of harm and the likely impact on these dimensions of particular interventions. It is then up to elected representatives to determine the priorities.