Harm reduction constitutes a significant approach which aims at decreasing the harmful consequences of drug use. The approach involves a range of measures including Methadone Maintenance Therapy (MMT) and Needle and Exchange Programme (NSEP) for lowering drug-related harms, particularly the Human Immunodeficiency Virus (HIV) infection. Considerable debate still exists, particularly in Malaysia on whether the local government should legitimise the harm reduction practice. The vehement criticism for the approach is that it is not actually effective in achieving the outcomes but merely implies an aid and promotion of drug consumption and hence should be banned. This article seeks to address the scientific bases for legitimising harm reduction approach. It examines the effectiveness of MMT and NSEP strategies on the subset of its outcomes, namely the decrease in HIV transmission and its scientific evidence. This article relies on data sourced from materials including published peer-reviewed research articles, books and reports. The analysis provided in this article suggests that the MMT and NSEP practice is justifiable based on its contribution to the control of HIV pathogen transmission. The available national and international evidence compellingly indicates that both interventions are effective in reducing HIV risk behaviours and the viral infection incidence among drug users. The outcome is unaffected by negative empirical data as there is a persuasive expounding explanation for them. The effectiveness of the interventions for reducing the blood-borne virus may be further increased by multi-integrated strategies. Moreover, considering scant local empirical findings, the efficacy of harm reduction interventions, particularly with respect to NSEP, warrants further research. It is critically vital for the scientific bases for harm reduction approach to be firmly sustained and articulated in law and policy discourse.