Many scientists believe that Prozac works by blocking
the action of the serotonin uptake pump.

When serotonin is released from the "sending" nerve cell,
some of it is reabsorbed by an uptake pump. By blocking the
serotonin uptake pump, Prozac increases the amount of active
serotonin that can be delivered to the "receiving" nerve cell.
This may help message transmission return to normal.