Antihistamine

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Antihistamine

An H₁ antagonist is a histamine antagonist

Histamine antagonist

A histamine antagonist is an agent which serves to inhibit the release or action of histamine. Antihistamine can be used to describe any histamine antagonist, but it is usually reserved for the H1 antagonist that act upon the histamine H1 receptor....
of the H₁ receptor that serves to reduce or eliminate effects mediated by histamine

Histamine

Histamine is a biogenic amine involved in local immune system as well as regulating physiological function in the gut and acting as a neurotransmitter....
, an endogenous chemical mediator released during allergic reactions

Allergy

Allergy is a Disorder of the immune system often also referred to as atopy. Allergic reactions occur to Natural environmental substances known as allergens; these reactions are Acquired disorder, predictable and rapid....
. Agents where the main therapeutic effect is mediated by negative modulation of histamine receptors are termed antihistamines - other agents may have antihistaminergic action but are not true antihistamines.

In common use, the term "antihistamine" refers only to H₁ antagonists, also known as H₁-receptor antagonists and H₁-antihistamines.

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Encyclopedia

An H₁ antagonist is a histamine antagonist

Histamine antagonist

Histamine

Allergy

Inverse agonist

In pharmacology, an inverse agonist is an agent which binds to the same receptor binding-site as an agonist for that receptor and reverses Receptor #Constitutive activity of receptors....
s at the histamine H₁-receptor, rather than antagonists

Receptor antagonist

A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a Receptor , but blocks or dampens agonist-mediated responses....
per se.

Pharmacology

In allergic reactions, an allergen

Allergen

An allergen is a parasite antigen capable of stimulating a type-I hypersensitivity reaction in atopy individuals.Most humans mount significant Immunoglobulin E responses only as a defense against parasitic infections....
(a type of antigen

Antigen

An antigen is a substance that prompts the generation of antibodies and can cause an immune response. The word originated from the notion that they can stimulate antibody generation....
) interacts with and cross-links surface IgE antibodies

Antibody

Antibodies are gamma globulin proteins that are found in blood or other bodily fluids of vertebrates, and are used by the immune system to identify and neutralize foreign objects, such as bacterium and viruses....
on mast cell

Mast cell

A mast cell is a resident cell of several types of tissues and contains many Granule rich in histamine and heparin. Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing and defense against pathogens....
s and basophils. Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell degranulation

Degranulation

Degranulation is a cellular process that releases antimicrobial cytotoxic molecules from secretion vesicle s called Granule found inside some cells....
and the release of histamine (and other chemical mediators) from the mast cell or basophil. Once released, histamine can react with local or widespread tissues through histamine receptor

Histamine receptor

The histamine receptors are a class of G-protein coupled receptors with histamine as their endogenous ligand.There are four known histamine receptors:...
s.

Histamine, acting on H₁-receptors, produces pruritus

Itch

Itch is an unpleasant sensation that evokes the desire or reflex to scratch. Itch has resisted many attempts to classify it as any one type of sensory experience....
, vasodilation

Vasodilation

Vasodilation refers to the widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls, particularly in the large arteries, smaller arterioles and large veins....
, hypotension

Hypotension

In physiology and medicine, hypotension refers to an abnormally low blood pressure. This is best understood as a physiologic state, rather than a disease....
, flushing

Flushing (physiology)

For a person to flush is to become markedly red in the face and often other areas of the skin, from various physiology conditions. Flushing is generally distinguished, despite a close physiological relation between them, from blushing, which is milder, generally restricted to the face or cheeks, and generally assumed to reflect embarrassment....
, headache

Headache

In medicine a headache or wiktionary:cephalalgia is a symptom of a number of different conditions of the head and sometimes neck. Some of the causes are benign while others are medical emergencies....
, tachycardia

Tachycardia

The word tachycardia comes from the Greek words tachys and kardia .Tachycardia typically refers to a heartrate that exceeds the range of the normal resting heartrate, based upon age:...
, bronchoconstriction

Bronchoconstriction

Bronchoconstriction is the constriction of the airways in the lungs due to the tightening of surrounding smooth muscle, with consequent coughing, wheezing, and shortness of breath....
, increase in vascular permeability

Vascular permeability

Vascular permeability characterizes the capacity of a blood vessel wall to pass through small molecules or even whole cells . Blood vessel walls are lined by a single layer of endothelial cells....
, potentiation of pain

Pain

Pain, in the sense of physical pain, is a typical sensory experience that may be described as the unpleasant awareness of a noxious stimulus or bodily harm....
, and more.

While H₁-antihistamines help against these effects, they work only if taken before contact with the allergen. In severe allergies, such as anaphylaxis

Anaphylaxis

Anaphylaxis is an acute Circulatory system and very severe Type I hypersensitivity allergy reaction in humans and other mammals. The term comes from the Greek words a?a ana and f??a??? phylaxis ....
or angioedema

Angioedema

Angioedema is the rapid swelling of the dermis, subcutaneous tissue, mucosa and submucosal tissues. It is very similar to urticaria, but urticaria occurs in the upper dermis....
, these effects may be so severe as to be life-threatening. Additional administration of epinephrine

Epinephrine

Epinephrine is a hormone and neurotransmitter.Epinephrine increases the "fight or flight" response of the Sympathetic nervous system of the autonomic nervous system....
, often in the form of an autoinjector (Epi-pen), is required by people with such hypersensitivities.

Clinical use of H₁-antihistamines

Indications

H₁-antihistamines are clinically used in the treatment of histamine-mediated allergic conditions. Specifically, these indications may include:

Allergic rhinitis
Rhinitis

Rhinitis, commonly known as a runny nose, is the medical term describing irritation and inflammation of some internal areas of the nose. The primary symptom of rhinitis is nasal...
Allergic conjunctivitis
Conjunctivitis

Conjunctivitis is an inflammation of the conjunctiva , most commonly due to an allergic reaction or an infection ....
Allergic dermatological
Skin

The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
conditions (contact dermatitis
Contact dermatitis

Contact dermatitis or Irritant dermatitis is a term for a skin reaction resulting from exposure to allergens or Irritation . Phototoxic dermatitis occurs when the allergen or irritant is activated by sunlight....
)
Urticaria
Urticaria

Urticaria are a kind of skin rash notable for dark red, raised, itchy bumps. Hives are frequently caused by allergic reactions, however there are many non-allergic causes....
Angioedema
Angioedema

Angioedema is the rapid swelling of the dermis, subcutaneous tissue, mucosa and submucosal tissues. It is very similar to urticaria, but urticaria occurs in the upper dermis....
Diarrhea
Pruritus (atopic dermatitis
Atopic dermatitis

Atopic dermatitis is an inflammation, chronically relapsing, non-contagious and pruritis skin disease. It has been given names like "prurigo Besnier," "neurodermitis," "endogenous eczema," "flexural eczema," "infantile eczema," and "prurigo diathsique"....
, insect bites)
Anaphylactic
Anaphylaxis

Anaphylaxis is an acute Circulatory system and very severe Type I hypersensitivity allergy reaction in humans and other mammals. The term comes from the Greek words a?a ana and f??a??? phylaxis ....
or anaphylactoid reactions - adjunct only
Nausea and vomiting (first-generation H₁-antihistamines)
Sedation (first-generation H₁-antihistamines)

H₁-antihistamines can be administered topically (through the skin

Skin

The skin is the outer covering of the body, also known as the epidermis. It is the largest organ of the integumentary system made up of multiple layers of epithelial biological tissue, and guards the underlying muscles, bones, ligaments and organ s....
, nose

Nose

Anatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for Respiration in conjunction with the mouth....
, or eye

Eye

Eyes are Organ that detect light, and send signals along the optic nerve to the visual system and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system....
s) or systemically, based on the nature of the allergic condition.

The authors of the American College of Chest Physicians Updates on Cough Guidelines (2006) recommend that, for cough associated with the common cold, first-generation antihistamine-decongestants are more effective than newer, non-sedating antihistamines. First-generation antihistamines include diphenhydramine

Diphenhydramine

Diphenhydramine hydrochloride , trade name Benadryl as produced by McNeil Laboratories a division of J&J, or Dimedrol outside the U.S....
(Benadryl); carbinoxamine (Clistin); clemastine (Tavist); chlorpheniramine (Chlor-Trimeton) and brompheniramine (Dimetane). However, it is important to note that a 1955 study of "antihistaminic drugs for colds," carried out by the U.S. Army Medical Corps, reported that "there was no significant difference in the proportion of cures reported by patients receiving oral antihistaminic drugs and those receiving oral placebos. Furthermore, essentially the same proportion of patients reported no benefit from either type of treatment."

Adverse drug reactions

Adverse drug reaction

An adverse drug reaction or adverse drug event is an expression that describes the unwanted, negative consequences associated with the use of given medications....
s are most commonly associated with the first-generation H₁-antihistamines. This is due to their relative lack of selectivity for the H₁-receptor.

The most common adverse effect is sedation; this "side-effect" is utilized in many OTC sleeping-aid preparations. Other common adverse effects in first-generation H₁-antihistamines include dizziness, tinnitus

Tinnitus

Tinnitus is the perception of sound within the human ear in the absence of corresponding external sound.Tinnitus can be perceived in one or both ears or in the head....
, blurred vision, euphoria

Euphoria (emotion)

Euphoria is medically recognized as an emotional and mental state defined as a sense of great happiness and quality_of_life. Technically, euphoria is an affect , but the term is often colloquially used to define emotion as an intense, Wiktionary:transcendent happiness combined with an overwhelming sense of well-being....
, uncoordination, anxiety

Anxiety

Anxiety is a psychological and physiological state characterized by cognitive, somatic, emotional, and behavioral components. These components combine to create an unpleasant feeling that is typically associated with uneasiness, fear, or worry....
, insomnia

Insomnia

Insomnia is a symptom of a sleep disorder characterized by persistent difficulty falling sleep or staying asleep despite the opportunity. Insomnia is a symptom, not a stand-alone diagnosis or a disease....
, tremor, nausea

Nausea

Nausea is the sensation of unease and discomfort in the stomach with an urge to vomit....
and vomiting, constipation

Constipation

Constipation, costiveness, or irregularity, is a condition of the digestive system in which a person experiences hard feces that are difficult to expel....
, diarrhea

Diarrhea

In medicine, diarrhea, also spelled diarrhoea , is characterized by frequent loose or liquid bowel movements. The spelling of "diarrhea" is an appropriation of the Greek "diarrhoia" meaning "a flowing through." ....
, dry mouth, and dry cough. Infrequent adverse effects include urinary retention, palpitation

Palpitation

A palpitation is an abnormal awareness of the heart rate of the heart, whether it is too slow, too fast, irregular, or at its normal frequency. It should not be confused with ectopic beat....
s, hypotension

Hypotension

In physiology and medicine, hypotension refers to an abnormally low blood pressure. This is best understood as a physiologic state, rather than a disease....
, headache

Headache

Hallucination

A hallucination, in the broadest sense, is a perception in the absence of a stimulus . In a stricter sense, hallucinations are defined as perceptions in a conscious and awake state in the absence of external stimuli which have qualities of real perception, in that they are vivid, substantial, and located in external objective space....
, and psychosis

Psychosis

Psychosis , with adjective psychotic, literally means abnormal condition of the mind, and is a generic psychiatry term for a mental state often described as involving a "loss of contact with reality"....
.

The newer second-generation H₁-antihistamines are far more selective for peripheral histamine H₁-receptors and have a far improved tolerability profile compared to the first-generation agents. The most common adverse effects noted for second-generation agents include drowsiness, fatigue, headache, nausea and dry mouth.

First-generation (non-selective, classical)

These are the oldest H₁-antihistaminergic drugs and are relatively inexpensive and widely available. They are effective in the relief of allergic symptoms, but are typically moderately to highly-potent muscarinic acetylcholine receptor

Acetylcholine receptor

An acetylcholine receptor is an integral membrane protein that responds to the binding of the neurotransmitter acetylcholine....
-antagonists (anticholinergic

Anticholinergic

An anticholinergic agent is a substance that blocks the neurotransmitter acetylcholine in the central nervous system and the peripheral nervous system....
) agents as well. These agents also commonly have action at a-adrenergic receptor

Adrenergic receptor

The adrenergic receptors are a class of G protein-coupled receptors that are targets of the catecholamines. Adrenergic Receptor s specifically bind and are activated by their endogenous ligands, the catecholamines adrenaline and noradrenaline ....
s and/or 5-HT receptor

5-HT receptor

In the field of neurochemistry, 5-HT receptors are receptor for the neurotransmitter and Periphery signal mediator serotonin, also known as 5-hydroxytryptamine or 5-HT....
s. This lack of receptor-selectivity is the basis of the poor tolerability-profile of some of these agents, especially compared with the second-generation H₁-antihistamines. Patient response and occurrence of adverse drug reactions vary greatly between classes and between agents within classes.

Classes

The first H₁-antihistamine discovered was piperoxan, by Ernest Fourneau and Daniel Bovet

Daniel Bovet

Daniel Bovet was a Swiss-born Italy pharmacology who won the 1957 Nobel Prize in Physiology or Medicine for his discovery of drugs that block the actions of specific neurotransmitters....
(1933) in their efforts to develop a guinea pig

Guinea pig

The guinea pig is a species of rodent belonging to the family Caviidae and the genus Cavia. Despite their common name, these animals are not pigs, nor do they come from Guinea ....
animal-model for anaphylaxis

Anaphylaxis

Nobel Prize in Physiology or Medicine

The Nobel Prize in Physiology or Medicine is awarded once a year by the Swedish Karolinska Institutet. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outstanding contributions in Nobel Prize in Physics, Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and Physiology or Medic...
for his contribution. Following their discovery, the first-generation H₁-antihistamines were developed in the following decades. They can be classified on the basis of chemical structure, and agents within these groups have similar properties.

Class	Description	Examples >- \| Ethylenediamines	Ethylenediamines were the first group of clinically-effective H₁-antihistamines developed.	Mepyramine Mepyramine Mepyramine is a Antihistamine#First-generation H1-receptor antagonists, targeting the histamine H1 receptor. However, it rapidly permeates the brain and so often causes drowsiness as a side effect.... (pyrilamine) Antazoline Antazoline Antazoline is an antihistamine used to relieve nasal congestion and in eye drops, usually in combination with naphazoline, to relieve the symptoms of allergy conjunctivitis.... \|- \| Ethanolamines	Diphenhydramine was the prototypical agent in this group. Significant anticholinergic Anticholinergic An anticholinergic agent is a substance that blocks the neurotransmitter acetylcholine in the central nervous system and the peripheral nervous system.... adverse effects, as well as sedation, are observed in this group but the incidence of gastrointestinal adverse effects is relatively low.	Diphenhydramine Diphenhydramine Diphenhydramine hydrochloride , trade name Benadryl as produced by McNeil Laboratories a division of J&J, or Dimedrol outside the U.S.... Carbinoxamine Carbinoxamine Carbinoxamine is an H1 class antihistamine. It was first launched in the United States by the McNeil Laboratories under the brand name Clistin.... Doxylamine Doxylamine Doxylamine succinate is one of the many sedating antihistamines used by itself as a short-term sedative, in combination with other drugs as a night-time common cold and allergy relief drug, and a preparation is prescribed in combination with vitamin B6 to prevent morning sickness in pregnancy women.... Clemastine Clemastine Clemastine, also known as meclastin, is an antihistamine drug. Unlike loratadine or fexofenadine, clemastine is a sedating antihistamine, however it exhibits fewer side effects than most of the widely used antihistamines.... Dimenhydrinate Dimenhydrinate Dimenhydrinate is an over-the-counter drug used to prevent nausea and motion sickness. It is marketed in Portugal as Viabom, but in prescription format.... \|- \| Alkylamines	The isomer Isomer In chemistry, isomers are compounds with the same molecular formula but different structural formulae. Isomers do not necessarily share similar properties unless they also have the same functional groups.... ism is a significant factor in the activity of the agents in this group. E-triprolidine, for example, is 1000-fold more potent than Z-triprolidine. This difference relates to the positioning and fit of the molecules in the histamine H₁-receptor binding site. Alkylamines are considered to have relatively fewer sedative and gastrointestinal adverse effects, but relatively greater incidence of paradoxical CNS Central nervous system The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms.... stimulation.	Pheniramine Pheniramine Pheniramine maleate is an antihistamine used to treat allergy conditions such as hay fever or urticaria. It has relatively strong sedative effects, and may sometimes be used off-label as an over-the-counter sleeping pill in a similar manner to other sedating antihistamines such as diphenhydramine.... Chlorphenamine Chlorphenamine Chlorphenamine or chlorpheniramine , commonly marketed as its salt chlorphenamine maleate , is a H1 antagonist#First-generation_.28non-selective.2C_classical.29 alkylamine antihistamine used in the prevention of the symptoms of allergy conditions such as rhinitis and urticaria.... (chlorpheniramine) Dexchlorpheniramine Dexchlorpheniramine Dexchlorpheniramine maleate is an antihistamine used to treat allergy conditions such as hay fever or urticaria. It is the pharmacologically active dextrorotatory isomer of chlorpheniramine and seems to possess no advantage over the more common parent drug.... Brompheniramine Brompheniramine Brompheniramine , commonly marketed as its salt brompheniramine maleate is an antihistamine drug of the propylamine class. It is commonly available Over-the-counter drug and is indicated for the treatment of the symptoms of the common cold and allergic rhinitis, such as runny nose, itchy eyes, watery eyes, and sneezing.... Triprolidine Triprolidine Triprolidine hydrochloride is an Over-the-counter drug antihistamine. It is used to combat the symptoms associated with allergy and is sometimes combined with other common cold medications designed to provide general relief for influenza-like symptoms.... \|- \| Piperazines	These compounds are structurally-related to the ethylenediamines and the ethanolamines, and produce significant anticholinergic Anticholinergic An anticholinergic agent is a substance that blocks the neurotransmitter acetylcholine in the central nervous system and the peripheral nervous system.... adverse effects. Compounds from this group are often used for motion sickness, vertigo, nausea, and vomiting. The second-generation H₁-antihistamine cetirizine Cetirizine Cetirizine hydrochloride , an antihistamine, is a major metabolite of hydroxyzine, and a racemic histamine H1 receptor inverse agonist used in the treatment of allergies, hay fever, angioedema, and urticaria.... also belongs to this chemical group.	Cyclizine Cyclizine Cyclizine is an antihistamine drug used to treat nausea, vomiting and dizziness associated with motion sickness, Vertigo and post-operative following administration of general anaesthesia and opioids.... Chlorcyclizine Chlorcyclizine Chlorcyclizine is a piperazine antihistamine.... Hydroxyzine Hydroxyzine Hydroxyzine is a first-generation antihistamine, of the piperazine class that is an histamine receptor antagonist. It was synthesised in the early 1950s and the medicinal formulation of this drug was announced in the 04 August 1956 issue of Chemistry Week.... Meclizine Meclizine Meclizine is an antihistamine considered to be an antiemetic. It is sold under the brand names of Bonine, Bonamine, Antivert and Postafen and is most commonly used to inhibit nausea and vomiting.... \|- \| Tricyclics and Tetracyclics	These compounds differ from the phenothiazine Phenothiazine Phenothiazine is the organic compound with the formula S2NH. This yellow tricyclic compound is soluble in acetic acid, benzene, and ether.... antipsychotic Antipsychotic Antipsychotics are a group of psychoactive drugs commonly but not exclusively used to treat psychosis, which is typified by schizophrenia. Over time a wide range of antipsychotics have been developed.... s in the ring-substitution and chain characteristics. (Nelson, 2002) They are also structurally-related to the tricyclic antidepressant Tricyclic antidepressant Tricyclic antidepressants are a class of antidepressant Medications first used in the 1950s. They are named after the drugs' molecular structure, which contains three rings of atoms .... s (and tetracyclics Tetracyclic antidepressant There are also several chemically unrelated tetracyclic antibiotics based on Tetracycline.A tetracyclic antidepressant is an antidepressant psychoactive drug from the tetracyclic drug group.... ), explaining the H₁-antihistaminergic adverse effects of those three drug classes and also the poor tolerability profile of tricyclic H₁-antihistamines. The second-generation H₁-antihistamine loratadine was derived from compounds in this group.	Promethazine Promethazine Promethazine is a first-generation histamine H1 receptor antagonist, antihistamine and antiemetic medication. It can also have strong sedative effects although it is rarely used specifically for this.... Alimemazine Alimemazine Alimemazine , also known as trimeprazine , commonly provided as a tartrate salt, is a phenothiazine derivative that is used as an antipruritic .... (trimeprazine) Cyproheptadine Cyproheptadine Cyproheptadine is an antihistamine/anticholinergic and antiserotonergic agent. It acts as a 5-HT receptor receptor antagonist and also blocks calcium channels.... Azatadine Azatadine Azatadine is an antihistamine.External links... Ketotifen Ketotifen Ketotifen fumarate is an H1-antihistamine/mast cell stabilizer available in two forms. In its ophthalmic form, it is used to treat allergic conjunctivitis, or the itchy Red eye s caused by allergy....

Common structural features

Two aromatic rings, connected to a central carbon, nitrogen or CO
Spacer between the central X and the amine, usually 2-3 carbons in length, linear, ring, branched, saturated or unsaturated
Amine is substituted with small alkyl groups, e.g., CH3

X = N, R1 = R2 = small alkyl groups
X = C
X = CO

Chirality at X can increase both the potency and selectivity for H1-receptors
For maximum potency, the two aromatic rings should be orientated in different planes

for example, tricyclic ring system is slightly puckered and the two aromatic rings lie in different geometrical planes, giving the drug a very high potency.

Second-generation and third-generation (selective, non-sedating)

Second-generation

Second generation H₁-antihistamines are newer drugs that are much more selective for peripheral H₁ receptors in preference to the central nervous system

Central nervous system

The central nervous system is the part of the nervous system that functions to coordinate the activity of all parts of the bodies of multicellular organisms....
histaminergic and cholinergic receptors. This selectivity significantly reduces the occurrence of adverse drug reactions compared with first-generation agents, while still providing effective relief of allergic conditions.

Systemic:

Acrivastine
Acrivastine

Acrivastine is a medication used for the treatment of allergies and hay fever. It is a second-generation H1-receptor antagonist antihistamine and works by blocking H1 histamine receptors....
Astemizole
Astemizole

Astemizole is a second generation antihistamine drug which has a long duration of action. Astemizole was discovered by Janssen Pharmaceutica in 1977....
Cetirizine
Cetirizine

Cetirizine hydrochloride , an antihistamine, is a major metabolite of hydroxyzine, and a racemic histamine H1 receptor inverse agonist used in the treatment of allergies, hay fever, angioedema, and urticaria....
Loratadine
Loratadine

Loratadine is a drug used to treat allergy, and marketed for its non-sedating properties. It is marketed by Schering-Plough and Shionogi in Japan under several trade names such as Claritin, Claritin-D, Claritine, Clarityn, Clarityne or Fristamin depending on the market; by Cadila as Lorfast; by Sandoz...
Mizolastine
Mizolastine

Mizolastine is a once daily, non-sedating antihistamine. Sold in the UK on prescriptions as Mizollen. It blocks histamine H1 receptors and is commonly fast-acting....
Terfenadine
Terfenadine

Terfenadine is an antihistamine formerly used for the treatment of allergy conditions. It was marketed under various brand names including Seldane in the United States, Triludan in the United Kingdom, and Teldane in Australia....

Topical:

Azelastine
Azelastine

Azelastine is an antihistamine and mast cell stabilizer available as a nasal spray for hay fever and as eye drops for allergic conjunctivitis....
Levocabastine
Levocabastine

Levocabastine is a selective antihistamine#Second-generation H1-receptor antagonists used for allergic conjunctivitis.Levocabastine was discovered at Janssen Pharmaceutica in 1979....
Olopatadine
Olopatadine

Olopatadine hydrochloride is an antihistamine and mast cell stabilizer, sold as a Prescription drug eye drop . It is used to treat itching associated with allergic conjunctivitis ....

Third-generation

Third-generation H₁-antihistamines are the active enantiomer

Enantiomer

In chemistry, an enantiomer is one of two stereoisomers that are Superpose complete mirror images of each other, much as one's left and right Chirality are "the same" but opposite....
(levocetirizine) or metabolite

Metabolite

Metabolites are the intermediates and products of metabolism. The term metabolite is usually restricted to small molecules. A primary metabolite is directly involved in normal growth, development, and reproduction....
(desloratadine & fexofenadine) derivatives of second-generation drugs intended to have increased efficacy

Efficacy

Efficacy is the capacity to produce an effect.It is these conditions that distinguish efficacy from the related concept of effectiveness, which relates to change under real-life conditions....
with fewer adverse drug reaction

Adverse drug reaction

An adverse drug reaction or adverse drug event is an expression that describes the unwanted, negative consequences associated with the use of given medications....
s. Indeed, fexofenadine is associated with a decreased risk of cardiac arrhythmia

Cardiac arrhythmia

Cardiac arrhythmia is a term for any of a large and heterogeneous group of conditions in which there is abnormal Electrical conduction system of the heart in the heart....
compared to terfenadine. However, there is little evidence for any advantage of levocetirizine or desloratadine, compared to cetirizine

Cetirizine

Cetirizine hydrochloride , an antihistamine, is a major metabolite of hydroxyzine, and a racemic histamine H1 receptor inverse agonist used in the treatment of allergies, hay fever, angioedema, and urticaria....
or loratadine

Loratadine

Loratadine is a drug used to treat allergy, and marketed for its non-sedating properties. It is marketed by Schering-Plough and Shionogi in Japan under several trade names such as Claritin, Claritin-D, Claritine, Clarityn, Clarityne or Fristamin depending on the market; by Cadila as Lorfast; by Sandoz...
, respectively.

There is some controversy associated with the use of this term.

Systemic:

Levocetirizine
Levocetirizine

Levocetirizine is a third generation non-sedative antihistamine, developed from the second generation antihistamine cetirizine. Chemically, levocetirizine is the active enantiomer of cetirizine....
Desloratadine
Desloratadine

Desloratadine is a drug used to treat allergy. It is marketed under several trade names such as NeoClarityn, Claramax, Clarinex and Aerius....
Fexofenadine
Fexofenadine

Fexofenadine hydrochloride is an antihistamine medication used in the treatment of hayfever and similar allergy symptoms. It was developed as a successor of and alternative to terfenadine , an antihistamine with potentially serious contraindications....

Antihistamine

Pharmacology

Clinical use of H1-antihistamines

Indications

Adverse drug reactions

First-generation (non-selective, classical)

Classes

Common structural features

Second-generation and third-generation (selective, non-sedating)

Second-generation

Third-generation

External links

Clinical use of H₁-antihistamines