Learn what ADHD looks like in adults and what you can do about it. or anxiety, were caused by other impairments that did not respond to usual treatment. It may be due to hyperactivity or sleep problems that can come with ADHD. Or it could be due to the constant effort to focus required by adults with ADHD. Or it could be a side effect of ADHD medications. Whatever the cause, fatigue can make attention difficulties even worse.Forgetfulness · Restlessness and anxiety · Fatigue.