Prozac is prescribed for the treatment of depression--that is, a continuing depression that interferes with daily functioning. The symptoms of major depression often include changes in appetite, sleep habits, and mind/body coordination; decreased sex drive; increased fatigue; feelings of guilt or worthlessness; difficulty concentrating; slowed thinking; and suicidal thoughts.

Prozac is also prescribed to treat obsessive-compulsive disorder. An obsession is a thought that won't go away; a compulsion is an action done over and over to relieve anxiety. The drug is also used in the treatment of bulimia (binge-eating followed by deliberate vomiting). It has also been used to treat other eating disorders and obesity.

Under the brand name Sarafem, the active ingredient in Prozac is also prescribed for the treatment of premenstrual dysphoric disorder (PMDD), formerly known as premenstrual syndrome (PMS). Symptoms of PMDD include mood problems such as anxiety, depression, irritability or persistent anger, mood swings, and tension. Physical problems that accompany PMDD include bloating, breast tenderness, headache, and joint and muscle pain. Symptoms typically begin 1 to 2 weeks before a woman's menstrual period and are severe enough to interfere with day-to-day activities and relationships.

Prozac is a member of the family of drugs called "selective serotonin re-uptake inhibitors." Serotonin is one of the chemical messengers believed to govern moods. Ordinarily, it is quickly reabsorbed after its release at the junctures between nerves. Re-uptake inhibitors such as Prozac slow this process, thereby boosting the levels of serotonin available in the brain.