Due to the lack of research into adults with ADHD, the symptoms of ADHD in adults are harder to define. Specialists have suggested that the way adults exhibit inattentiveness, hyperactivity and impulsivity can differ from that of children. For example, inattentiveness seems to worsen parallel to the pressures that increase in adult life, but hyperactivity tends to decrease. As it is a developmental disorder, it has been suggested that ADHD cannot appear in adults without first presenting in some way during childhood. As a result, sometimes the symptoms of ADHD in children and teenagers are applied to adults potentially displaying ADHD, although perhaps more subtly.

The following have been proposed as symptoms associated with ADHD in adults:

• Difficulty focusing or prioritising 
• Lack of organisational skills 
• Unable to keep quiet and interrupting others 
• Forgetfulness 
• Misplacing items constantly 
• Irritability, short temper, and mood swings 
• Impatience that seems excessive 
• No sense of danger or personal safety for themselves or others
• Carelessness; Restlessness