Ranking evidence in substance use and addiction.

Evidence-based medicine has consistently prized the epistemological value of randomized-controlled trials (RCTs) owing to their methodological advantages over alternative designs such as observational studies. However, there are limitations to RCTs that hinder their ability to study chronic and dynamic conditions such as substance use and addiction. For these conditions, observational studies may provide superior evidence based on methodological and practical strengths. Assuming epistemic superiority of RCTs has led to an inappropriate devaluation of other study designs and the findings they support, including support for harm reduction services, especially needle exchange programs and supervised injection facilities. The value offered by observational studies should be reflected in evidence-based medicine by allowing more flexibility in evidence hierarchies that presume methodological superiority of RCTs. Despite the popularity of evidence ranking systems and hierarchies, nothing should replace critical appraisal of study methodology and examining the suitability of applying a given study design to a specific research question.