An antihistamine is a drug which serves to reduce or eliminate effects mediated by histamine, an endogenous chemical mediator released during allergic reactions, through action at the histamine receptor. Only 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-receptor antagonists, also known as HIncluding:

• Antihistamine - Pharmacology
• Antihistamine - Clinical use of antihistamines
• Antihistamine - Indications
• Antihistamine - Adverse drug reactions
• Antihistamine - First-generation H₁-receptor antagonists
• Antihistamine - Ethylenediamines
• Antihistamine - Ethanolamines
• Antihistamine - Alkylamines
• Antihistamine - Piperazines
• Antihistamine - Tricyclics
• Antihistamine - Common structural features of classical antihistamine
• Antihistamine - Second-generation H₁-receptor antagonists
• Antihistamine - Systemic
• Antihistamine - Topical
• Antihistamine - Common structural features of non-sedating antihistamines
• Antihistamine - Third-generation H₁-receptor antagonists
• Antihistamine - Systemic
• Antihistamine - Other agents
• Antihistamine - Inhibitors of histamine release
• Antihistamine - H₂-receptor antagonists
• Antihistamine - H₃- and H₄-receptor antagonists
• Antihistamine - Other agents with antihistaminergic activity

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In allergic reactions an allergen (a type of antigen) interacts with and cross-links surface IgE antibodies on mast cells and basophils. Once the mast cell-antibody-antigen complex is formed, a complex series of events occurs that eventually leads to cell-degranulation 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 receptors. Histamine, acting on H1-receptors, produces pruritis, vasodilatation, hypotension, flushing, headache, tachycardia, bronchoconstriction, increases vascular permeabili ...

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Antihistamine, Antihistamine - Pharmacology, Antihistamine - Clinical use of antihistamines, Antihistamine - Indications, Antihistamine - Adverse drug reactions, Antihistamine - First-generation H₁-receptor antagonists, Antihistamine - Ethylenediamines, Antihistamine - Ethanolamines, Antihistamine - Alkylamines, Antihistamine - Piperazines, Antihistamine - Tricyclics, Antihistamine - Common structural features of classical antihistamine, Antihistamine - Second-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Topical, Antihistamine - Common structural features of non-sedating antihistamines, Antihistamine - Third-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Other agents, Antihistamine - Inhibitors of histamine release, Antihistamine - H₂-receptor antagonists, Antihistamine - H₃- and H₄-receptor antagonists, Antihistamine - Other agents with antihistaminergic activity

Read more here: » Antihistamine: Encyclopedia II - Antihistamine - Pharmacology

These are the oldest 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-antagonists (anticholinergic) agents as well. These agents also commonly have action at α-adrenergic receptors and/or 5-HT receptors. 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 a ...

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Antihistamine, Antihistamine - Pharmacology, Antihistamine - Clinical use of antihistamines, Antihistamine - Indications, Antihistamine - Adverse drug reactions, Antihistamine - First-generation H₁-receptor antagonists, Antihistamine - Ethylenediamines, Antihistamine - Ethanolamines, Antihistamine - Alkylamines, Antihistamine - Piperazines, Antihistamine - Tricyclics, Antihistamine - Common structural features of classical antihistamine, Antihistamine - Second-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Topical, Antihistamine - Common structural features of non-sedating antihistamines, Antihistamine - Third-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Other agents, Antihistamine - Inhibitors of histamine release, Antihistamine - H₂-receptor antagonists, Antihistamine - H₃- and H₄-receptor antagonists, Antihistamine - Other agents with antihistaminergic activity

Read more here: » Antihistamine: Encyclopedia II - Antihistamine - First-generation H₁-receptor antagonists

Antihistamine - Inhibitors of histamine release. These agents appear to stabilise the mast cells to prevent degranulation and mediator release. cromoglicate (cromolyn) nedocromil Antihistamine - H2-receptor antagonists. Main article: H2-receptor antagonist Clinically-relevant histamine H2-receptors are found principally in the parietal cells of the gastric mucosa. H2-receptor "antagonists" are also inve ...

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Antihistamine, Antihistamine - Pharmacology, Antihistamine - Clinical use of antihistamines, Antihistamine - Indications, Antihistamine - Adverse drug reactions, Antihistamine - First-generation H₁-receptor antagonists, Antihistamine - Ethylenediamines, Antihistamine - Ethanolamines, Antihistamine - Alkylamines, Antihistamine - Piperazines, Antihistamine - Tricyclics, Antihistamine - Common structural features of classical antihistamine, Antihistamine - Second-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Topical, Antihistamine - Common structural features of non-sedating antihistamines, Antihistamine - Third-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Other agents, Antihistamine - Inhibitors of histamine release, Antihistamine - H₂-receptor antagonists, Antihistamine - H₃- and H₄-receptor antagonists, Antihistamine - Other agents with antihistaminergic activity

Read more here: » Antihistamine: Encyclopedia II - Antihistamine - Other agents

These are newer drugs that are much more selective for peripheral H1 receptors in preference to the central nervous system 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. Antihistamine - Systemic. acrivastine astemizole cetirizine loratadine mizolastine terfenadine (withdrawn from ...

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Antihistamine, Antihistamine - Pharmacology, Antihistamine - Clinical use of antihistamines, Antihistamine - Indications, Antihistamine - Adverse drug reactions, Antihistamine - First-generation H₁-receptor antagonists, Antihistamine - Ethylenediamines, Antihistamine - Ethanolamines, Antihistamine - Alkylamines, Antihistamine - Piperazines, Antihistamine - Tricyclics, Antihistamine - Common structural features of classical antihistamine, Antihistamine - Second-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Topical, Antihistamine - Common structural features of non-sedating antihistamines, Antihistamine - Third-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Other agents, Antihistamine - Inhibitors of histamine release, Antihistamine - H₂-receptor antagonists, Antihistamine - H₃- and H₄-receptor antagonists, Antihistamine - Other agents with antihistaminergic activity

Read more here: » Antihistamine: Encyclopedia II - Antihistamine - Second-generation H₁-receptor antagonists

Antihistamine - Indications. H1-antihistamines are clinically used in the treatment of histamine-mediated allergic conditions. Specifically, these indications may include: (Rossi, 2004) allergic rhinitis allergic conjunctivitis allergic dermatological conditions (contact dermatitis) urticaria angioedema pruritus (atopic dermatitis, insect bites) anaphylactic or anaphylactoid reactions - adjunct only nausea and vomiting (first-generation H1-antihistamines)

These are the active enantiomer (levocetirizine, desloratadine) or metabolite (fexofenadine) derivatives of second-generation drugs intended to have increased efficacy with fewer adverse drug reactions. Indeed, fexofenadine is associated with a decreased risk of cardiac arrhythmia compared to terfenadine. However, there is little evidence for any advantage of levocetirizine or desloratadine, compared to cetirizine or loratadine respectively. Antihistamine - SystemicSee also:

Antihistamine, Antihistamine - Pharmacology, Antihistamine - Clinical use of antihistamines, Antihistamine - Indications, Antihistamine - Adverse drug reactions, Antihistamine - First-generation H₁-receptor antagonists, Antihistamine - Ethylenediamines, Antihistamine - Ethanolamines, Antihistamine - Alkylamines, Antihistamine - Piperazines, Antihistamine - Tricyclics, Antihistamine - Common structural features of classical antihistamine, Antihistamine - Second-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Topical, Antihistamine - Common structural features of non-sedating antihistamines, Antihistamine - Third-generation H₁-receptor antagonists, Antihistamine - Systemic, Antihistamine - Other agents, Antihistamine - Inhibitors of histamine release, Antihistamine - H₂-receptor antagonists, Antihistamine - H₃- and H₄-receptor antagonists, Antihistamine - Other agents with antihistaminergic activity

Read more here: » Antihistamine: Encyclopedia II - Antihistamine - Third-generation H₁-receptor antagonists