What are SSRIs and why are they the first-line treatment for depression?

Selective serotonin reuptake inhibitors, or SSRIs, block the reuptake of serotonin in the synaptic cleft, increasing serotonin availability in the brain. Common SSRIs include fluoxetine, sertraline, paroxetine, citalopram, and escitalopram. They are first-line for depression because they are effective, well-tolerated, and have a relatively safe side effect profile compared to older antidepressants. Common side effects include nausea, headache, sexual dysfunction, insomnia, and weight changes. Full therapeutic effect takes four to six weeks.