drugs used to treat depression, anxiety disorders, obsessive-compulsive disorder, and posttraumatic stress disorder. (Several widely used antidepressant drugs are selective serotonin reuptake inhibitors—SSRIs.) - Many of these drugs work by increasing the availability of neurotransmitters, such as norepinephrine or serotonin, which elevate arousal and mood and appear scarce when a person experiences feelings of depression or anxiety