Antidepressant capsules. Box and blister pack of Prozac capsules. Each capsule contains 20 milligrams of the antidepressant drug fluoxetine hydrochloride. This belongs to a class of drugs known as selective serotonin reuptake inhibitors (SSRIs). Serotonin is a chemical which mediates moods in the brain. It is thought a lack of it can lead to depression. Prozac works by stopping existing serotonin in the brain being destroyed, prolonging its active life. SSRIs have fewer and less serious side-effects than the older antidepressant drugs, which are far less selective in their actions. Antidepressant drugs may take several days to show any effects.