ADHD is a mental health condition that is defined through analysis of behaviour. People with ADHD show a persistent pattern of inattention and/or hyperactivity–impulsivity that interferes with day-to-day functioning and/or development.

The impact of ADHD differs with age reflecting both the changing maturity of the individual and the changing circumstances and expectations on them. As they mature, an individual may be better able to cope but they may continue to struggle due to the increased behavioural expectations on them. For those with hyperactivity, a child may command incessant and demanding extremes of activity; then as an adolescent moving to fidgeting instead of larger movements and as an adult having a sustained inner sense of restlessness. Inattention may diminish in absolute terms, and attention span normally improves with age, but it usually lags behind that of unaffected people, and behind the level that is expected and needed for everyday attainments [NICE guidelines p16]

There is no one cause of ADHD. It is considered to be a result of an often complex interplay between genetic and environmental factors – with genetic factors being responsible for 70%-80% of the probability.

Extract from ADHD UK