Research on treatments for patients with co-occurring psychiatric and substance use disorders is of core importance and at the same time highly challenging as it includes patients that are normally excluded from clinical studies. Such research may require methodological adaptations which in turn create new challenges. However, the challenges that arise in such studies are insufficiently discussed in the literature. The aim of this methodology paper is, firstly, to discuss the methodological adaptations that may be required in such research; secondly, to describe how such adaptations created new challenges in a group-randomized clinical trial on Integrated Treatment amongst patients with co-occurring psychiatric and substance use disorders. We also discuss how these challenges might be understood and highlight lessons for future research in this field. Trial registration: NCT00447733.
Clinical trial implementation and recruitment: Lessons learned from the early closure of a randomized clinical trial
