Background: Involving adults with cognitive impairments, and specifically intellectual disability, in research is critical to developing appropriate and effective interventions but is highly challenging. Our aim was to examine where complexities lie in delivering research in underrepresented and hard to reach populations using an exemplar process evaluation conducted as part of a drug reduction trial. Methods: Quantitative methods were used to assess recruitment, adherence to the intervention and safety data. Qualitative interviews examined non-efficacy based barriers to drug reduction in clinical practice. Results: Feasibility of carrying out a drug reduction trial was limited by a lack of exploration of acceptability. Barriers to successful delivery included concerns around wider care team co-operation and consent procedures. Conclusions: It is important to consider interventions involving adults with cognitive impairment, and particularly intellectual disability, as complex. Current process evaluation frameworks require further adaptation to guide research and innovation in these populations.