Background. Research shows that 91.2% of inpatients at forensic psychiatric wards have a history of drug use and a third of those patients use drugs frequently. To be able to confront and address patients with substance abuse on the ward, the working staff needs to have adequate competence concerning the ability to understand who the patients are. Aim. The aim of this study was to explore how the working staff use their competence when interacting with patients who are influenced by drugs on forensic psychiatric wards. Method. The study was conducted in the western parts of Sweden and at three forensic psychiatric wards. The study was designed after a qualitative design and based on an empirical approach with interviews. The data was collected by interviewing a total of six informants through semi-structured interviews. The interviews were analysed using a qualitative content analysis. Results. The result ended up in eight themes, showing that informants in this study used their competence to recognize substance abuse, to help the patients navigating around potential dangers and create a caring relation. Further they trusted their experience, but all informants requested further education. Conclusion. The informants in this study expressed that they had good strategies in terms of supporting and rehabilitating patients with substance abuse. All informants requested further education about preventing relapse, knowledge about drug abuse and knowing how to work after a relapse.