It is very common for someone who has ADHD to be diagnosed with another psychiatric condition. Approximately 70% of adults with ADHD are diagnosed with depression at some point during their life.
Often, the symptoms of one condition mask the presence of the other. For example, a person could be diagnosed with depression and given antidepressants to treat it but then find that they still struggle with inattentiveness. They may then be diagnosed with ADHD as well. ADHD was present the whole time but was initially hidden by the symptoms of depression, which can be very similar to the symptoms of the predominantly inattentive ADHD type.
As well as depression, other conditions that often occur with ADHD include anxiety, bipolar disorder, Tourette’s syndrome, dyslexia and sensory processing disorder.