SSRIs (Selective Serotonin Reuptake Inhibitors) are a class of antidepressant medications that work by increasing serotonin levels in the brain. They were first marketed in the late 1990s and are commonly prescribed to treat conditions like depression and anxiety by selectively inhibiting the reabsorption of serotonin. SSRIs are considered a first-line treatment for many patients with depression or anxiety disorders.