An introduction and overview of the mental health conditions relevant to people with intellectual disability. The chapter focuses on the evidence base to support or refute whether they suffer greater rates of mental health problems, Psychiatric classification and prescribing; Comparison tables of international classification of diseases (ICD) versions 10 and 11; and the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders (DSM) 4 and 5 and DC-LD.

The International Classification of Diseases ICD-11 describes a block called ‘Schizophrenia spectrum and other primary psychiatric disorders’ which includes schizophrenia, schizoaffective disorder, schizotypal disorder, acute and transient psychotic disorder, delusional disorder and other specified schizophrenias or other primary psychotic disorders. All these conditions are characterised by impaired assessment of reality and behaviour, delusions, hallucinations, disorganised thinking and behaviour, experiences of passivity and control, negative symptoms, and psychomotor disturbances. The ICD-11 specifies a symptom duration of at least one month and has removed the reliance on Schneiderian first-rank symptoms, giving equal weight to any hallucinations or delusion. Schizophrenia and other psychotic disorders form part of the group of severe mental illness. They can prove difficult to assess and treat in people with intellectual disability. The chapter presents an overview of the condition, the treatments with medication available, and their relevance.

Personality refers to the innate and enduring characteristics that influence an individual’s attitudes, behaviours, and experience of themselves, others, and the world. Historically, personality disorders were conceptualised as enduring and pervasive disturbance in an individual’s patterns of thinking, feelings, and behaviours. This disturbance results in significant disturbance in their psychosocial functioning and interpersonal relationships The diagnosis of personality disorders in people with intellectual disability can be a contentious issue. The chapter presents an overview of the condition, the treatments with medication available, and their relevance.

Anxiety disorders are common mental health problems and include panic disorder, generalised anxiety disorder, post-traumatic stress disorder, and obsessive compulsive disorder. The chapter presents an overview of the condition, the treatments and medication available, and their relevance to people with intellectual disability.

According to the ICD-11 (World Health Organization, 2019), substance use disorders include disorders that result from a single occasion or repeated use of substances (both legal and illegal) that have psychoactive properties; all drugs that are taken in excess have in common direct activation of the brain reward system, which is involved in the reinforcement of behaviours and the production of memories. They produce such an intense activation of the reward system that normal activities may be neglected. Instead of achieving reward system activation through adaptive behaviours, drugs of abuse directly activate the reward pathways. The pharmacological mechanisms by which each class of drugs produces reward are different, but the drugs typically activate the system and produce feelings of pleasure, often referred to as a ‘high’ (American Psychiatric Association, 2013). This chapter will cover all substance use in people with intellectual disability, whether it is a problem, and alternative treatments.