Prozac is an SSRI; Selective Serotonin Re-uptake Inhibitor. When the brain produces too much serotonin (a neuro-transmiter) certain brain cells re-uptake (re-absorb) the excess. SSRIs are designed to block the re-uptake process. They work very well for a lot of people, but they aren't really exactly sure how it works. On the other hand they make other people worse, especially in teenagers, they can make them suicidal. If you don't really need them it's best to stay away from them.