From the Guidelines
Antihistamines exhibit their therapeutic effect by blocking activity at histamine receptor sites. This is the primary mechanism by which antihistamines prevent histamine from binding and causing allergy symptoms 1. When allergens trigger an immune response, mast cells release histamine, which normally binds to these receptors and causes inflammation, itching, sneezing, and other allergic reactions.
Key Points
- Antihistamines work by blocking histamine receptors, primarily the H1 receptors, to prevent histamine from binding and causing allergy symptoms 1.
- First-generation antihistamines like diphenhydramine (Benadryl) cross the blood-brain barrier and can cause drowsiness, while second-generation options like cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra) are less sedating 1.
- Typical dosing for adults includes cetirizine 10mg daily, loratadine 10mg daily, or fexofenadine 180mg daily, with relief provided for 12-24 hours depending on the specific drug 1.
- For optimal effectiveness, antihistamines work best when taken regularly before exposure to allergens rather than after symptoms develop, due to their competitive binding mechanism 1.
Mechanism of Action
The competitive binding mechanism of antihistamines explains why they don't immediately stop symptoms once histamine has already bound to receptors, making preventive use more effective than reactive use 1. Antihistamines act as inverse agonists at histamine receptors, effectively reducing the symptoms associated with histamine release, such as pruritus, flushing, and urticaria 1.
Clinical Considerations
Given the potential side effects and interactions of antihistamines, particularly first-generation options, it's crucial to consider the patient's overall health, potential drug interactions, and the specific symptoms being treated when selecting an antihistamine 1. The choice between first-generation and second-generation antihistamines should be based on the patient's needs, including the presence of sedation as a desirable or undesirable effect, and the potential for anticholinergic side effects 1.
From the FDA Drug Label
PURPOSE Antihistamine Uses • Temporarily relieves these symptoms due to hay fever or other respiratory allergies: Antihistamine
The correct answer is b. Blocking activity at histamine receptor sites.
- Antihistamines work by blocking the action of histamine at receptor sites, which helps to relieve symptoms such as runny nose, sneezing, and itchy eyes. 2 3 2
From the Research
Mechanism of Antihistamines
- Antihistamines exhibit their therapeutic effect by blocking the activity at histamine receptor sites 4, 5, 6, 7, 8
How Antihistamines Work
- They act principally to prevent histamine-receptor interaction through competition with histamine for histamine receptors 4
- Individual antihistaminic drugs act to inhibit histaminic action at one or another histamine receptor (H1 or H2-receptor), but not at both receptors 4
- Antihistamines are helpful therapeutically in preventing, rather than reversing, histaminic actions 4
Types of Antihistamines
- H1-receptor inhibitors are most effective therapeutically in inhibiting manifestations of histamine-induced wheal and erythema formation and pruritus 4
- H2-receptor inhibitors are able to inhibit histamine-induced gastric acid secretion 4
Therapeutic Uses
- Antihistaminic drugs are helpful in the control of allergic disorders, allergic rhinitis and urticaria in particular 4, 8
- Newly developed H2-receptor inhibitors show therapeutic promise in the treatment of peptic ulceration 4
- Antihistamines are useful in the treatment of allergic conditions, Parkinson's disease, insomnia and some forms of nausea, and provide symptomatic relief of cough and other conditions associated with respiratory tract infections 5