Prozac capsules. Prozac is a proprietary brand of antidepressant drug. The active part is fluoxetine hydrochloride, which acts as a selective serotonin reuptake inhibitor (SSRI). This inhibition causes an increase in levels of serotonin, the chemical in the brain that is responsible for feelings of happiness and well-being. Prozac is only available on prescription. These are 20mg capsules, usually taken daily. Prozac is marketed by the Eli Lilly pharmaceutical company.