When a person takes this drug, some of those drugs include things like Prozac or Zoloft... When people take those, more serotonin hangs out in the synapse and is able to be taken up by the neuron on the opposite side because of this selective reuptake inhibition. It prevents the clearance of serotonin from the synapse, and thereby more serotonin can have an effect. So, SSRIs don't increase the total amount of serotonin in the brain; they change how effective the serotonin that's already in the brain is at changing the activity of neurons.