Older people living in care homes (long-term care facilities) are at high risk of developing infections due to weakened immunity, close proximity living, and other multiple illnesses. As a result, care home residents are prescribed far more antibiotics than the general population. Probiotics are friendly live bacteria that may give health benefits by improving the immune system of care home residents and reducing the spread of harmful bacteria. We conducted a randomized controlled trial to investigate whether a probiotic supplement could reduce the number of infections in care home residents, reduce antibiotic use in this vulnerable group, and so help curb antibiotic resistance. Conducting clinical trials in care homes can be challenging, in part because a high proportion of residents have conditions such as dementia that may affect their ability to provide informed consent to take part. When someone lacks capacity to provide consent for themselves, alternative processes are followed to ensure that the research is conducted ethically. This case study provides an account of the challenges of conducting a randomized controlled trial in care homes, focusing on the recruitment of residents who lack capacity to consent, and strategies we employed to ensure their appropriate inclusion in the trial.