SSRI
An SSRI, or selective serotonin reuptake inhibitor, is the most widely prescribed type of antidepressant. Imagine a serotonin signal as a puddle of messenger left in the gap between two neurons. Normally a pump quickly mops it back up. An SSRI plugs that pump, so the serotonin signal lingers and the receiving neuron 'hears' it for longer.
Specifically, SSRIs block the serotonin transporter (SERT) on the presynaptic neuron, preventing reuptake of serotonin from the synaptic cleft. They are called 'selective' because they act mainly on serotonin with little effect on norepinephrine or dopamine transporters, which gives them a cleaner side-effect profile than older tricyclic antidepressants. Common examples are fluoxetine, sertraline, citalopram, escitalopram, and paroxetine.
SSRIs are first-line for depression and for a broad range of anxiety disorders, panic disorder, OCD, PTSD, and premenstrual dysphoric disorder. As with all antidepressants, full benefit takes weeks.
Typical side effects include nausea, headache, sexual dysfunction, and early jitteriness; abrupt stopping can cause a discontinuation syndrome. They are much safer in overdose than tricyclics, but combining them with other serotonergic drugs risks serotonin syndrome — agitation, fever, tremor, and autonomic instability.
Sertraline is often chosen for a patient with both depression and anxiety because SSRIs treat both, and it is reasonably safe even if taken in overdose.
The favourable safety margin makes SSRIs a common first choice.
SSRIs are a specific subgroup of reuptake inhibitors aimed at the serotonin transporter; SNRIs additionally block norepinephrine reuptake, and older tricyclics hit several targets at once, which is why tricyclics cause more side effects.