The aim of the study was to identify the causes of relapse among substance abusers after rnsubstance use disorder treatment. A qualitative, exploratory case study research design was used rnfor the study. A total of six patients with relapse case under treatment and two health professionals rnwere selected using purposive sampling technique. In-depth interviews and key informant rninterview were employed to collect data. In using this technique, it was supplemented by inclusive rncriteria to make the selection free of the researcherâ€™s bias. The data were analyzed using thematic rnanalysis. The site of the study was St. Paul Millennium medical college rehabilitation center and rnAmanuel mental specialized hospital rehabilitation center. The findings show that psychosocial rneffects like negative emotional states, lack of support, and peer pressures are recounted in factors rncausing relapse. Environmental factor and Economic problems are the other relevant causes of rnrelapse revealed by the study. The finding also revealed that, health, psychology, socialrnrelationship and economy were affected by relapse. It is recommended that structured, holistic rnaftercare should be implemented. Effective relapse prevention should be made by understanding rncauses of relapse. Based on the findings of the study, future research areas and effective rninterventions were forwarded. There are only few researches done on the causes of relapse in rnEthiopian context, so this research will contribute essential information for future researchers