Antihistamine

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.

Clinical use of antihistamines

Indications

H1-antihistamines are clinically used in the treatment of histamine-mediated allergic conditions. Specifically, these indications may include: (Rossi, 2004)

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• 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)
• sedation (first-generation H₁-antihistamines)

Antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition.

Related Topics:
Skin - Nose - Eye

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Adverse drug reactions

Adverse drug reactions are most commonly associated with the first-generation H1-antihistamines. This is due to their relative lack of selectivity for the H1-receptor.

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The most common adverse effect is sedation - this "side effect" being utilised in many OTC sleeping-aid preparations. Other common adverse effects in first-generation H1-antihistamines include: dizziness, tinnitus, blurred vision, euphoria, uncoordination, anxiety, insomnia, tremor, nausea and vomiting, constipation, diarrhoea, dry mouth, and dry cough. Infrequent adverse effects include: urinary retention, palpitations, hypotension, headache, hallucination, and psychosis. (Rossi, 2004)

Related Topics:
OTC - Tinnitus - Euphoria - Anxiety - Insomnia - Nausea - Constipation - Diarrhoea - Palpitation - Hypotension - Headache - Hallucination - Psychosis

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The newer second-generation H1-antihistamines are far more selective for peripheral histamine H1-receptors and, correspondingly, 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. (Rossi, 2004)

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