The vast majority of people with bipolar disorder and about 35% of those with major depressive disorder have at least one other psychiatric illness. Some of these illnesses include anxiety disorders, eating disorders, substance use disorders, obsessive compulsive disorder (OCD) and attention deficit hyperactivity disorder (ADHD). ADHD can be difficult to diagnose because symptoms can overlap with other disorders. A group of researchers has found that adults with bipolar disorder and unipolar disorder experience ADHD at rates significantly higher than the average population.