Prozac is an antidepressant in a group of drugs called selective serotonin reuptake inhibitors . Prozac affects chemicals in the brain which may become unbalanced and cause depression, panic, anxiety, or obsessive-compulsive symptoms. Prozac is used to treat major depressive disorder, bulimia nervosa (an eating disorder) obsessive-compulsive disorder, panic disorder, and premenstrual dysphoric disorder (PMDD). Prozac is sometimes used together with another medication called olanzapine (Zyprexa) to treat depression caused by bipolar disorder (manic depression). This combination is also used to treat depression after at least 2 other medications have been tried without successful treatment of symptoms.