Antihistamine Classes, Examples and Therapeutic Uses

Q — Classify antihistaminic drugs and their therapeutic uses.

Classification of Antihistamines and Therapeutic Uses

1. Introduction to Antihistamines

Antihistamines are drugs that block or reduce the effects of histamine, a chemical involved in allergic reactions, gastric acid secretion, and neurotransmission. They are primarily classified based on the histamine receptor subtype they act on:

• H₁ receptor antagonists (H₁ antihistamines)

• H₂ receptor antagonists (H₂ blockers)

• H₃ and H₄ antagonists (experimental/limited clinical use)

2. Classification of Antihistamines

A. H₁ Receptor Antagonists (H₁ Blockers)

1. First-Generation H₁ Antihistamines

• Cross the blood–brain barrier → cause sedation

• Have anticholinergic and antiemetic effects

Examples:

• Diphenhydramine

• Chlorpheniramine

• Hydroxyzine

• Promethazine

• Dimenhydrinate

• Meclizine

2. Second-Generation H₁ Antihistamines

• Do not cross the blood–brain barrier significantly → non‑sedating

• More selective for peripheral H₁ receptors

Examples:

• Loratadine

• Cetirizine

• Fexofenadine

• Desloratadine

• Levocetirizine

B. H₂ Receptor Antagonists (H₂ Blockers)

• Inhibit gastric acid secretion by blocking H₂ receptors in gastric parietal cells

Examples:

• Ranitidine (withdrawn in many countries)

• Famotidine

• Cimetidine

• Nizatidine

C. H₃ and H₄ Receptor Antagonists

• H₃ blockers: Investigated for neurological disorders (e.g., narcolepsy, Alzheimer’s disease)

• H₄ blockers: Investigational use in inflammatory and immune diseases

Examples: (mostly experimental)

• Thioperamide (H₃)

• JNJ-7777120 (H₄)

3. Therapeutic Uses of Antihistamines

A. H₁ Antihistamines

1. Allergic conditions

• Allergic rhinitis (hay fever)

• Urticaria (hives)

• Allergic conjunctivitis

• Atopic dermatitis

• Anaphylaxis (as an adjunct to epinephrine)

2. Respiratory disorders

• Asthma (adjunct)

• Common cold (symptomatic relief)

3. Motion sickness and nausea

• Meclizine, promethazine, dimenhydrinate

4. Insomnia

• Diphenhydramine, hydroxyzine (short-term use)

5. Vertigo

• Meclizine, promethazine

6. Pruritus and insect bites

B. H₂ Antihistamines

1. Peptic ulcer disease

• Duodenal and gastric ulcers

2. Gastroesophageal reflux disease (GERD)

3. Zollinger–Ellison syndrome

4. Stress ulcers (prevention in ICU patients)

Summary Table

Would you like a diagram or chart to help visualize this classification?