Brands, Medical Use, Clinical Data
Drug Category
- Anti-Allergic Agents
- Antipruritics
- Histamine H1 Antagonists
Dosage Forms
- Syrup (0.5 mg drug per 5mL syrup) and tablet (oral, 1mg)
Brands / Synonyms
Clemastina [INN-Spanish]; Clemastine; Clemastine Fumarate; Clemastine [USAN:BAN]; Clemastinum [INN-Latin]; Meclastin; Mecloprodin; Tavist; Tavist-1
Indications
For the relief of symptoms associated with allergic rhinitis such as sneezing, rhinorrhea, pruritus and acrimation.
Pharmacology
Clemastine is an antihistamine with anticholinergic (drying) and sedative side effects. Antihistamines competitively antagonize various physiological effects of histamine including increased capillary permeability and dilatation, the formation of edema, the "flare" and "itch" response, and gastrointestinal and respiratory smooth muscle constriction. Within the vascular tree, H1- receptor antagonists inhibit both the vasoconstrictor and vasodilator effects of histamine. Depending on the dose, H1- receptor antagonists can produce CNS stimulation or depression. Most antihistamines exhibit central and/or peripheral anticholinergic activity. Antihistamines act by competitively blocking H1- receptor sites. Antihistamines do not pharmacologically antagonize or chemically inactivate histamine, nor do they prevent the release of histamine.
Mechanism of Action
Clemastine is a selective histamine H1 antagonist and binds to the histamine H1 receptor. This blocks the action of endogenous histamine, which subsequently leads to temporary relief of the negative symptoms brought on by histamine.
Absorption
Rapidly absorbed from the gastrointestinal tract.
Toxicity
Oral LD50 in rat and mouse is 3550 mg/kg and 730 mg/kg, respectively. Antihistamine overdosage reactions may vary from central nervous system depression to stimulation. In children, stimulation predominates initially in a syndrome which may include excitement, hallucinations, ataxia, incoordination, muscle twitching, athetosis, hyperthermia, cyanosis convulsions, tremors, and hyperreflexia followed by postictal depression and cardio-respiratory arrest. Convulsions in children may be preceded by mild depression. Dry mouth, fixed dilated pupils, flushing of the face, and fever are common. In adults, CNS depression, ranging from drowsiness to coma, is more common.
Biotrnasformation / Drug Metabolism
Antihistamines appear to be metabolized in the liver chiefly via mono- and didemethylation and glucuronide conjugation.
Contraindications
Antihistamines are contraindicated in patients hypersensitive to the drug or to other antihistamines of similar
chemical structure. Antihistamines should not be used in newborn or premature infants. Because of the higher risk of
antihistamines for infants generally and for newborns and prematures in particular, antihistamine therapy is
contraindicated in nursing mothers.
Drug Interactions
Additive CNS depression may occur when antihistamines are administered concomitantly with other CNS depressants
including barbiturates, tranquilizers, and alcohol. Patients receiving antihistamines should be advised against the
concurrent use of other CNS depressant drugs.
Monoamine oxidase (MAO) inhibitors prolong and intensify the anticholinergic effects of antihistamines.
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