WHAT IS KNOWN ON THE SUBJECT?: Mental health service users with medical co-morbidity frequently experience difficulties accessing and receiving appropriate treatment in emergency departments. Service users frequently experience fragmented care planning and coordinating between tertiary medical and primary care services. Little is known about mental health nurses' perspectives about how to address these problems. WHAT THIS PAPER ADDS TO EXISTING KNOWLEDGE?: Emergency department clinicians' poor communication and negative attitudes have adverse effects on service users and the quality of care they receive. The findings contribute to the international evidence about mental health nurses' perspectives of service users feeling confused and frustrated in this situation, and improving coordination and continuity of care, facilitating transitions and increasing family and caregiver participation. Intervention studies are needed to evaluate if adoption of these measures leads to sustainable improvements in care planning and coordination, and how service users with medical co-morbidity are treated in emergency departments in particular. WHAT ARE THE IMPLICATIONS FOR PRACTICE?: Effective planning and coordination of care are essential to enable smooth transitions between tertiary medical (emergency departments in particular) and primary care services for service users with medical co-morbidity. Ongoing professional development education and support is needed for emergency department clinicians. There is also a need to develop an organized and systemic approach to improving service users' experience in emergency departments., Abstract: Introduction Mental health service users with medical co-morbidity frequently experience difficulties accessing appropriate treatment in medical hospitals, and often there is poor collaboration within and between services. Little is known about mental health nurses' perspectives on how to address these problems. Aim To explore mental health nurses' perspectives of the experience of service users with medical co-morbidity in tertiary medical services, and to identify how to improve care planning and coordination for service users transitioning between tertiary medical and primary care services. Method Embedded within an experience-based co-design study, focus group discussions were conducted with 17 emergency department nurses and other clinicians, in Melbourne, Australia. Results Three main themes were abstracted from the data: feeling confused and frustrated, enhancing service users' transition and experience and involving families and caregivers. Participants perceived the service user experience to be characterized by fear, confusion and a sense of not being listened to. They highlighted that service users' transition and experience could be enhanced by facilitating transitions and improving coordination and continuity of care. They also emphasized the need to increase family and caregiver participation. Conclusion Our findings contribute to knowledge about improving the way service users are treated in emergency departments and improving care planning and coordination; in particular, facilitating transitions, improving coordination and continuity of care and increasing family and caregiver participation., (© 2016 The Authors. Journal of Psychiatric and Mental Health Nursing Published by John Wiley & Sons Ltd.)