Abstract Introduction With the emergence of the COVID-19 epidemic, most European countries implemented measures to prevent the spread of infections inside prison, including: use of personal protective equipment (PPE), hygiene practices, limits to mobility, reduction of overcrowding. Their implementation affected the drug related interventions. The EMCDDA has conducted a rapid assessment to evaluate the impact of those measures on the provision of drug treatment and harm reduction services in the European prisons of 15 countries between March and June 2020. Methods The study was based on a mixed method approach that triangulated the results of an on-line survey, the outcome of a focus group with prison and drugs experts and case studies from four European countries. Results and discussion Most countries reported a reduction in the provision of drug treatment and, to a lesser extent, of harm reduction interventions inside prison during the early phase of the pandemic. The drug services tried to adapt to the new context. Innovations were introduced, including the use of telemedicine in counselling and pharmacological treatment, a better partnership between security and health staff and a more individualised approach to drug treatment. Concerns were expressed around reduction of some interventions, such as group-based interventions, services provided by external agencies, interventions in preparation for release, continuity of care and possible reduction in testing and treatment of drug related infections such as HIV, HCV and HBV due to the priority given to the COVID-19 emergence. Conclusions With the emergence of COVID-19, drug services had to address various challenges, making efforts to maintain the provision of drug-related interventions inside prison, while introducing the COVID-19 containment measures. Ensuring the equivalence and continuity of health care provision for those in prison with drug problem remains of central importance.