Major depressive disorder (MDD) is a mood disorder characterized by persistent feelings of sadness and loss of interest that interferes with normal daily functioning.
The exact chemical basis for depression is poorly understood but is thought to be linked to decreased levels of the neurotransmitters, serotonin, dopamine, and norepinephrine. This cause is also known as the monoamine hypothesis.
Because of this, the drugs developed to treat MDD selectively target the reuptake of serotonin and norepinephrine, increasing levels of these neurotransmitters in the synapse and enhancing NT signaling.
SSRIs are generally first-line due to their improved tolerability compared to the others (TCAs and MOAIs) and relative safety in cases of overdose.
Studies have shown equivalent efficacy among antidepressants, therefore the initial choice is made empirically based on patient factors and the nuances of each drug.