The interface between the Mental Capacity Act 2005 and the Mental Health Act 1983 can be complex, particularly in patients with co-existing mental and physical illnesses. The management of these patients requires the involvement of patients, relatives and multidisciplinary teams. This article presents four illustrative patient cases, all of whom suffered from co-existing mental and physical illnesses. In managing these cases, dilemmas had arisen in the provision of treatment encompassing both legal frameworks. These cases helped to emphasise the decision-specific and time-specific nature of assessment of mental capacity, requiring clinicians to assess on a case-by-case basis over a suitable period. Often, principles from both legal frameworks may be applied by the treatment team. These cases help to highlight the significant overlap between mental and physical health, which often cannot be managed independently. This may call for the need to better integrate the current legal frameworks, and the optimal involvement of specialists across both settings.