Prevention of HIV is one of the more persuasive public health arguments in favour of opioid substitution treatment. However, until now, there has been no quantitative estimate of the extent to which this treatment reduces the transmission of HIV. This knowledge gap has been filled by the findings of a linked systematic review by MacArthur and colleagues (doi:10.1136/bmj.e5945).1 This analysis of data from nine studies that included 819 incident HIV infections over 23 608 person years of follow-up found that treatment with methadone was associated with a 54% reduction in the risk of HIV infection among people who inject drugs.

An earlier systematic review found that opioid substitution treatment with methadone or buprenorphine is associated with reductions in behaviours associated with a high risk of HIV transmission,2 but owing to methodological limitations, overall estimates of the extent of reduction of these risk behaviours were not calculated. Individual studies were consistent in their findings of reductions of illicit opioid use (ranging from 32% to 69%), injecting drug use (20-60%), and sharing of injecting equipment (25-86%). Treatment …