40 years of overdose prevention centres: what works, for whom and why

What is it about?

Overdose prevention centres (OPCs) are facilities where people can consume illicit drugs they have already obtained, under the supervision of trained staff who can respond to overdoses. The first officially sanctioned OPC opened in Switzerland in 1986, and they have since spread across Europe, North America, Latin America and Australia. This realist review synthesised 391 documents to produce the first full programme theory of how OPCs work, including a diagrammatic logic model mapping the relationships between contexts, mechanisms and outcomes. The review identifies three specific causal pathways through which OPCs produce benefits for people in differing circumstances related to housing status, gender identity and ethnicity, and local drug market conditions. Key interventions include providing safe and hygienic consumption spaces, overdose response, harm reduction education, and protection from street and gender-based violence, which trigger mechanisms of safety, trust, social inclusion, autonomy and empowerment. The review also maps a "dark logic" model of conditions under which OPCs fail to produce intended outcomes (including not involving people who use drugs in the decision making process).

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Why is it important?

Previous reviews of OPCs have established that they work, demonstrating reductions in overdose deaths, HIV and hepatitis C transmission, ambulance call-outs and public drug use. This review goes further by explaining why and how they work, which is essential knowledge for designing, implementing and evaluating new services. That causal understanding is urgently needed: the UK and several other countries have only recently opened or approved their first OPCs, synthetic high-strength opioids including nitazenes are emerging as a new threat in Europe, and North America is experiencing devastating fentanyl-driven overdose mortality. The full programme theory, complete logic model, and identification of pathways for specific population groups, including people experiencing homelessness, women and gender-diverse people, and other communities, gives policymakers and service providers an evidence base for maximising benefits.

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