Antidepressant capsule. Fingers holding a capsule printed with DISTA 3105. The capsule is Prozac Pulvule, which is manufactured by Eli Lilly and Company. It 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 that 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. Antidepressant drugs may take several days to show any effects.