There is no one-size-fits-all answer to this question, as the brain chemical lacking in ADHD may vary from person to person. However, some of the most common brain chemicals that may be deficient in people with ADHD include dopamine, serotonin, and norepinephrine. Dopamine is responsible for feelings of pleasure and satisfaction and is often deficient in people with ADHD. This may contribute to the Impulsive and Hyperactive symptoms of ADHD, as people with low dopamine may be more likely to act on their impulses without thinking about the consequences. Serotonin is responsible for feelings of happiness and well-being and is often deficient in people with ADHD. This may contribute to the Inattentive symptoms of ADHD, as people with low serotonin may be less motivated to focus on tasks that don't bring them happiness. Norepinephrine is responsible for feelings of alertness and focus, and it is often deficient in people with ADHD. This may contribute to the Inattentive and Hyperactive symptoms of ADHD, as people with low norepinephrine may be less alert and unable to focus on tasks.