Background The United Kingdom (UK) is currently experiencing a public health crisis of drug-related deaths. The government has rejected recommendations to open overdose prevention services, under the Misuse of Drugs Act 1971. To report on the operation and use of an unsanctioned overdose prevention service which operated in Glasgow city centre from September 2020 to May 2021. Methods Description of the service, with analysis of data collected on its use. Results The service operated for nine months without permission or funding from official sources. We report on the 894 injections supervised and recorded, and nine successful interventions with overdose events (seven opioid/two cocaine). Powder cocaine injection predominated either alone (60.6%) or with heroin (22.1%). Injection was mostly in the groin (68.0%) or arm (16.8%). More injections were recorded by males (70.1%). Around 65% of injection events featured an individual who was on a buprenorphine/methadone prescription. Conclusion It is feasible for an overdose prevention service to operate successfully in the UK without being shut down by the police or with negative consequences for the community. Future sites in the UK must tailor to the substances used by their potential clients, international trends (e.g. for fentanyl use) did not apply here. There is an urgent need and demand for these services in the UK to reduce harm, prevent and intervene during overdose, and provide vital psychosocial support for health and wellbeing in a highly marginalised population.