Dr Carnwath was until 2012 part-time Consultant in Addiction Psychiatry in Darlington, working for Tees, Esk & Wear NHS Trust. He retired from full-time NHS practice in 2008. He has continued doing part-time locum jobs up until the present day.
He has been a Consultant Psychiatrist since 1983, and has specialist qualification in both General Adult and Addiction Psychiatry. He has worked as a Consultant in Somerset, in Manchester and most recently in County Durham. Initially he worked mostly in General Adult Psychiatry and Rehabilitation. From about 1990 he shared his time between General Adult Psychiatry and Addiction Psychiatry. Since 2002 he has worked primarily in Addiction Psychiatry, but has continued to treat mentally ill patients both in hospital and the community
He has also trained as a GP, and is a Fellow of the Royal College of General Practitioners.
He has served on a number of National Advisory Committees, which have led to the production of reports, and have influenced national practice. Between 2002 and 2008 he was Honorary Senior Lecturer in Addiction at Imperial College, London. He has written about forty scientific papers relating to psychiatry or addiction, and has presented frequently as invited speaker at national and international conferences.