Zoloft is a type of antidepressant known as a selective serotonin reuptake inhibitor, or SSRI. It was the first in its class of drugs to be approved by the FDA, and Apo-metronidazole flagyl has been prescribed to over 50 million people since its release in 1992. The active ingredient in Zoloft modulates the action of serotonin (a naturally occurring chemical in your brain that influences your mood) on brain cells. This helps improve symptoms of depression (such as sadness and low energy), panic disorder , obsessive compulsive disorder, social anxiety disorder , and premenstrual dysphoric disorder .