Clemastine, also known as meclastin, is a first-generation H1 histamine antagonist (

antihistamine Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provide ...

) with anticholinergic properties (drying) and sedative side effects. Like all first-generation antihistamines, it is sedating. Patented in 1960, it came into medical use in 1967.

Medical uses

Clemastine is used to relieve hay fever and

allergy Allergies, also known as allergic diseases, refer a number of conditions caused by the hypersensitivity of the immune system to typically harmless substances in the environment. These diseases include hay fever, food allergies, atopic derm ...

symptoms, including sneezing; runny nose; and red, itchy, tearing eyes. Prescription strength clemastine is also used to relieve the itching and swelling of

hives Hives, also known as urticaria, is a kind of skin rash with red, raised, itchy bumps. Hives may burn or sting. The patches of rash may appear on different body parts, with variable duration from minutes to days, and does not leave any long-last ...

Side effects

Overdosage symptoms are paradoxical, ranging from CNS depression to stimulation. Stimulation is most common in children, and is usually followed by excitement, hallucinations,

ataxia Ataxia is a neurological sign consisting of lack of voluntary coordination of muscle movements that can include gait abnormality, speech changes, and abnormalities in eye movements. Ataxia is a clinical manifestation indicating dysfunction of ...

, loss of coordination,

muscle twitching A fasciculation, or muscle twitch, is a spontaneous, involuntary muscle contraction and relaxation, involving fine muscle fibers. They are common, with as many as 70% of people experiencing them. They can be benign, or associated with more serio ...

, athetosis,

hyperthermia Hyperthermia, also known simply as overheating, is a condition in which an individual's body temperature is elevated beyond normal due to failed thermoregulation. The person's body produces or absorbs more heat than it dissipates. When extrem ...

cyanosis Cyanosis is the change of body tissue color to a bluish-purple hue as a result of having decreased amounts of oxygen bound to the hemoglobin in the red blood cells of the capillary bed. Body tissues that show cyanosis are usually in locations ...

, convulsions, tremors, and hyperreflexia. This may be followed by postictal depression and cardiovascular/respiratory arrest. Other common overdose symptoms include dry mouth, fixed dilated pupils, flushing of the face, and

pyrexia Fever, also referred to as pyrexia, is defined as having a temperature above the normal range due to an increase in the body's temperature set point. There is not a single agreed-upon upper limit for normal temperature with sources using va ...

. In adults, overdose usually leads to CNS depression, ranging from drowsiness to

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhi ...

. Continuous and/or cumulative use of anticholinergic medications, including first-generation antihistamines, is associated with higher risk of cognitive decline and dementia in older people.

Pharmacology

Clemastine is an antihistamine with anticholinergic and

sedative A sedative or tranquilliser is a substance that induces sedation by reducing irritability or excitement. They are CNS depressants and interact with brain activity causing its deceleration. Various kinds of sedatives can be distinguished, but ...

effects. Antihistamines competitively bind to

histamine Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discover ...

receptor sites, thus reducing the neurotransmitter's effects. Effects of histamine (which are countered by antihistamines) include: * Increased capillary permeability * Increased capillary dilatation *

Edema Edema, also spelled oedema, and also known as fluid retention, dropsy, hydropsy and swelling, is the build-up of fluid in the body's tissue. Most commonly, the legs or arms are affected. Symptoms may include skin which feels tight, the area ma ...

(i.e., swelling) * Pruritus (Itch) * Gastrointestinal/respiratory smooth muscle constriction Clemastine inhibits both the

vasoconstrictor Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vess ...

and vasodilator effects of histamine. Depending on the dose, the drug can produce paradoxical effects, including CNS stimulation or depression. Most antihistamines exhibit some type of anticholinergic activity. Antihistamines act by competitively binding to H₁-receptor sites, thus blocking the binding of endogenous histamine. Antihistamines do not chemically inactivate or prevent the normal release of histamine. Clemastine does also act as FIASMA (functional inhibitor of acid sphingomyelinase). Clemastine is rapidly absorbed from the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

and peak plasma concentrations are attained in 2–4 hours. Antihistamines are thought to be metabolized in the liver, mostly by mono-/didemethylation and glucuronide conjugation. It is an inhibitor of cytochrome P450 CYP2D6 and may interfere with other drugs metabolized by this isozyme.

Mechanism of action

Clemastine is a selective histamine H₁ antagonist. It binds to the histamine H₁ receptor, thus blocking the action of

endogenous Endogenous substances and processes are those that originate from within a living system such as an organism, tissue, or cell. In contrast, exogenous substances and processes are those that originate from outside of an organism. For example, ...

histamine, which leads to temporary relief of the negative symptoms caused by histamine.

Society and culture

Clemastine is an OTC drug, and is available under many names and dosage forms worldwide. Most common brand name is Tavegyl.drugs.co
Clemastine at drugs.com international listings
Page accessed May 10, 2015

References

External links

The pharmacokinetics and bioavailability of clemastine and phenylpropanolamine in single-component and combination formulations
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Pharmacodynamics Pharmacodynamics (PD) is the study of the biochemical and physiologic effects of drugs (especially pharmaceutical drugs). The effects can include those manifested within animals (including humans), microorganisms, or combinations of organisms ...

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