Tricyclic antidepressants (TCAs) are a class of psychoactive drugs used primarily as antidepressants which were first introduced in the early 1950s. They are named after their chemical structure, which contains three rings of atoms.
They are closely related to the tetracyclic antidepressants (TeCAs), which contain four rings of atoms.
In recent times, the TCAs have been largely replaced in clinical use in most parts of the world by newer antidepressants such as the selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), among others, though they are still sometimes prescribed for certain indications.