The mechanism of action of TCAs relates to the inhibition of the reuptake of both serotonin and norepinephrine from the synaptic cleft of neurons in the central nervous system. Inhibition of reuptake has an acute onset; however, the desired clinical effects can take weeks. Thus, TCAs likely induce postsynaptic neuron regulatory changes that ultimately provide their clinical benefits. In addition, TCAs have anticholinergic effects and inhibitory effects at histamine and α₁-adrenergic receptors, which explains the TCA side-effect profile.

Compared with other antidepressants (SSRIs or monoamine oxidase inhibitors [MAOIs]), TCAs carry a greater risk of death due to overdose. TCAs have more than twice the risk of death associated with overdoses of MAOIs and more than five times the risk associated with SSRIs and atypical antidepressant medications. Of TCA overdoses, 2% to 3% result in death, and most of these are due to cardiac complications. The clinical manifestations of TCA overdose include psychomotor depression, seizures, tachycardia, and cardiac conduction defects (specifically, prolonged PR, QRS, and QT intervals). In addition, the anticholinergic properties of TCAs will induce dry skin and mucous membranes, blurred vision, flushing, urinary retention, constipation, and potentially autonomic instability—most notably hypotension.