Final answer:
Drugs that block the reuptake of serotonin increase the concentration of serotonin molecules in the synaptic gaps.
Step-by-step explanation:
Drugs that block the reuptake of serotonin increase the concentration of serotonin molecules in the synaptic gaps. These drugs, known as selective serotonin reuptake inhibitors (SSRIs), prevent serotonin from being taken back up into the presynaptic neuron, allowing it to remain in the synaptic gaps for a longer period of time. This leads to an increase in serotonin concentration, which can have a positive effect on conditions such as depression.