- BACKGROUND: Older adults in care homes have increasingly complex health care needs, and care provision should be evidence-based whenever possible. However, recruitment of frail, older people to research is a complex process and often results in care home residents being excluded from research participation. This paper draws on the experience of setting up a randomised controlled trial to determine the effectiveness of probiotics on antibiotic-associated diarrhoea in care home residents [Probiotics for Antibiotic Associated Diarrhoea in Care Homes (PAAD) Study] in Wales. FINDINGS: Significant challenges were encountered setting up a clinical trial in care homes. There were a number of barriers and facilitative factors encountered that were unique to this research setting. The classification of the study intervention (a widely available food supplement with a low risk safety profile) as an investigational medicinal product, with the associated requirements including obtaining statutory approvals and research governance, had a major impact. CONCLUSION: The process for setting up a clinical trial of an investigational medicinal product in care homes has been more complex and time consuming than the process for setting up an observational study in the same setting, and clinical trials in other health care settings. We recommend regulatory changes to ensure approvals processes are more proportionate to risk and context, to ensure that care home residents have the opportunity to participate in research and are able to help generate much needed evidence to underpin care. Recommendations made may inform future research practice. TRIAL REGISTRATION: ISRCTN 25324586.