Dual diagnosis usually refers to mental illness combined with substance misuse. But it may also be used to describe a number of other conditions, including physical health problems. In the UK social care sector, the term is sometimes used for people who have both a learning disability and a mental illness.

A combination of severe illness and problematic drug or alcohol use. These two things are closely linked for many people. People with a dual diagnosis often have serious physical, social and psychological problems.

NICE definition is broader than TLAP definition, which implies that drug or alcohol abuse is always a factor.