Prozac stops serotonin from being absorbed by the brain too quickly, making it so that your synaptic receptors have more time to absorb it. That gives you a more steady amount of serotonin, which is part of why Prozac is prescribed for depression. Too little serotonin makes you unhappy and apparently overtime it affects your motor control (people with Parkinson's apparently have declining levels of dopamine and serotonin). So from a logical standpoint, if Prozac helps you "keep serotonin available" in the brain, it *shouldn't* decrease your motor control and by that increase your stutter. But stuttering is not only an issue of motor control, and it could be that in some people Prozac affects other parts of the brain that process language planning and other systems related to stuttering.