trade name of fluoxetine hydrochloride, first of the class of antidepressant medications called selective serotonin reuptake inhibitors (SSRIs). It was introduced by Eli Lilly pharmaceutical company as a treatment for clinical depression in 1986. Prozac is also used to treat a variety of other psychiatric disorders, including obsessive-compulsive disorder and bulimia nervosa. The drug apparently achieves its therapeutic effect by interfering with the reabsorption of the neurotransmitter serotonin within the brain. Because SSRIs only inhibit the reuptake of serotonin, they have fewer, less-serious side effects than other antidepressants, which interfere with several neurotransmitter systems. Side effects include decreased sexual drive or ability, diarrhea, insomnia, headache, and nausea. The full therapeutic effect of Prozac may not be achieved until the drug has been taken for several weeks. Most physicians prescribe Prozac for at least six months to prevent a patient's symptoms from recurring.