Drug Groups

approved

Description

An ethanolamine-derivative, first generation histamine H1 antagonist used in hay fever, rhinitis, allergic skin conditions, and pruritus. It causes drowsiness. [PubChem]

Indication

For the relief of symptoms associated with allergic rhinitis such as sneezing, rhinorrhea, pruritus and acrimation. Also for the management of mild, uncomplicated allergic skin manifestations of urticaria and angioedema. Used as self-medication for temporary relief of symptoms associated with the common cold.

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.

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.

Affected Organisms

Biotransformation

Antihistamines appear to be metabolized in the liver chiefly via mono- and didemethylation and glucuronide conjugation.

Absorption

Rapidly absorbed from the gastrointestinal tract.

Elimination

Urinary excretion is the major mode of elimination.

References

External Links