PurposeThe aim of this study is to explore and discuss key challenges associated with having stakeholders take part in co-designing a health care intervention to increase mobility in older medical patients admitted to two medical departments at two hospitals in Denmark.Design/methodology/approachThe study used a qualitative design to investigate the challenges of co-designing an intervention in five workshops involving health professionals, patients and relatives. “Challenges” are understood as “situations of being faced with something that needs great mental or physical effort in order to be done successfully and therefore tests a person's ability” (Cambridge Dictionary). Thematic content analysis was conducted with a background in the analytical question: “What key challenges arise in the material in relation to the co-design process?”.FindingsTwo key challenges were identified: engagement and facilitation. These consisted of five sub-themes: recruiting patients and relatives, involving physicians, adjusting to a new researcher role, utilizing contextual knowledge and handling ethical dilemmas.Research limitations/implicationsThe population of patients and relatives participating in the workshops was small, which likely affected the co-design process.Practical implicationsResearchers who want to use co-design must be prepared for the extra time required and the need for skills concerning engagement, communication, facilitation, negotiation and resolution of conflict. Time is also required for ethical discussions and considerations concerning different types of knowledge creation.Originality/valueEngaging stakeholders in co-design processes is increasingly encouraged. This study documents the key challenges in such processes and reports practical implications.