Tricyclic antidepressants (TCAs) are heterocyclic chemical compounds used primarily as antidepressants. The TCAs were first discovered in the early 1950s and were subsequently introduced later in the decade; they are named after their chemical structure, which contains three rings of atoms. The tetracyclic antidepressants (TeCAs), which contain four rings of atoms, are a closely related group of antidepressant compounds. 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), which typically have more favourable side-effects profiles, though they are still sometimes prescribed for certain indications.