Tetracyclic antidepressants (TeCAs) are a class of antidepressants that were first introduced in the 1970s. They are named after their tetracyclic chemical structure, containing four rings of atoms, and are closely related to the tricyclic antidepressants (TCAs), which contain three rings of atoms.

TeCAs have diverse pharmacology and differ from TCAs in a number of ways. With the exception of amoxapine, TeCAs do not inhibit the reuptake of serotonin^{[citation needed]}. However, aside from mirtazapine, they do inhibit the reuptake of norepinephrine^{[citation needed]}. TeCAs block the serotonin 5-HT₂ receptors similarly to TCAs. Besides mirtazapine, they also block the α₁-adrenergic receptor^{[citation needed]}. Conversely, whereas TCAs have relatively low affinity for the α₂-adrenergic receptor, mianserin and mirtazapine potently antagonize this receptor, and this action is thought to be involved in their antidepressant effects^{[citation needed]}. TeCAs block the histamine H₁ receptor similarly to the TCAs, but tend to be even stronger antihistamines than TCAs^{[citation needed]}. On the other hand, in contrast to almost all TCAs, TeCAs have only low affinity for the muscarinic acetylcholine receptors, and for this reason, are associated with few or no anticholinergic side effects^{[citation needed]}. Mianserin and mirtazapine are far less toxic than TCAs in overdose.^[1][2]

• List of antidepressants
• Noradrenergic and specific serotonergic antidepressant