SSRI is an acronym for Selective Serotonin Reuptake Inhibitor. SSRIs are antidepressants  that work by inhibiting serotonin (a.k.a. 5-hydroxytryptamine or 5-HT) re-uptake. Unlike tricyclic antidepressants (TCAs) and serotonin/norepinephrine reuptake inhibitors (SNRIs), SSRIs have greater selectivity for the serotonin transporter than to norepinephrine transporter . Therefore, SSRIs are more specific, inhibiting only serotonin reuptake . This in turn will increase the serotonin level across the synaptic cleft.
SSRIs can have a variety of side effects on a patient, including drowsiness, nausea, insomnia, dry mouth and agitation.