Important differential diagnoses that must be considered include other neurodegenerative disorders, psychosis associated with the various causes of delirium, major depressive disorder, side effects of dopaminergic medications (especially dopamine agonists, COMT inhibitors), side effects of other medications (e.g. opiates, anticholinergics), hyponatremia, substance abuse or intoxication. This evidence concerns the gene COMT and major depressive disorder.