Tricyclics are cyclic chemical compounds that contain three fused rings of atoms.

Many compounds have a tricyclic structure, but in pharmacology, the term has traditionally been reserved to describe heterocyclic drugs. They include antidepressants, antipsychotics, anticonvulsants, and antihistamines (as antiallergens, anti-motion sickness drugs, antipruritics, and hypnotics/sedatives) of the dibenzazepine, dibenzocycloheptene, dibenzothiazepine, dibenzothiepin, phenothiazine, and thioxanthene chemical classes, and others.

• Promethazine and other first generation antihistamines with a tricyclic structure were discovered in the 1940s.
• Chlorpromazine, derived from promethazine originally as a sedative, was found to have neuroleptic properties in the early 1950s, and was the first typical antipsychotic.
• Imipramine, originally investigated as an antipsychotic, was discovered in the early 1950s, and was the first tricyclic antidepressant.
• Carbamazepine was discovered in 1953, and was subsequently introduced as an anticonvulsant in 1965.
• Clozapine, a derivative of imipramine, was synthesized in 1958 and entered the European market in 1972 under the name Leponex.
• Antidepressants with a tetracyclic structure such as mianserin and maprotiline were first developed in the 1970s as tetracyclic antidepressants.
• Loratadine was introduced as a non-sedating second generation antihistamine in the 1990s.^[1]

• Bisulepin
• Tetracyclic
• Tricyclic tropane analogs
• Heterocyclic