The interface between the Mental Health Act 1983 (MHA) and the Mental Capacity Act 2005 (MCA) can throw up complex issues. This article reviews a Court of Protection case that considered this interface specifically in the context of physical healthcare for a patient detained under the MHA. The court analysis also included consideration of the European Convention on Human Rights, the Mental Health Units (Use of Force) Act 2018 and the concept of residual liberty. The judgment describes principles to be applied when considering whether non-consensual physical health treatment for detained patients could result in a further deprivation of liberty. Discussion of other Court of Protection cases considers the issue of communication during a capacity assessment, the MHA/MCA interface in obstetric cases and what test to apply in determining whether a patient could be detained under the MHA. Such principles and guidance are helpful in clinical practice for healthcare professionals who deal with the MHA/MCA interface.