Final answer:
An SSRI (Selective Serotonin Reuptake Inhibitor) is a type of drug that inhibits the serotonin transporter, thereby increasing the concentration of serotonin in the synaptic cleft to treat depression and anxiety disorders.
Step-by-step explanation:
The substance that inhibits the serotonin transporter is known as a Selective Serotonin Reuptake Inhibitor (SSRI). SSRIs are a class of drugs typically used to treat depression and anxiety disorders by blocking the reuptake of the neurotransmitter serotonin into the presynaptic neuron. This action results in an increase in serotonin levels within the synaptic cleft, thus enhancing its activity in the brain. Some well-known SSRIs include fluoxetine (Prozac), sertraline (Zoloft), and paroxetine (Paxil). These medications are generally favored over older classes of antidepressants because they have a higher toxic dose threshold and milder side effects, making them safer for patients.