Anti-Histamine Drugs Lecture Notes

Comprehensive vocabulary flashcards covering the types, mechanisms, classifications, therapeutic uses, and adverse effects of anti-histamine drugs.

Histamine

A chemical messenger found in all tissues (especially lungs, skin, blood vessels, and GI tract) that is stored in mast cells and basophils; it functions in allergic/inflammatory reactions, gastric acid secretion, and CNS neurotransmission.

Histidine decarboxylase

The enzyme that converts the amino acid histidine into histamine.

H₁ Receptor Effects

Mediates smooth muscle contraction (bronchoconstriction), increased capillary permeability leading to edema, vasodilation of small vessels via nitric oxide (NO) release, and sensory nerve stimulation causing itching and pain.

H₂ Receptor Effects

Found mainly in gastric parietal cells, where they mediate increased gastric acid secretion.

First-Generation H₁ Blockers

Older, inexpensive drugs (e.g., Diphenhydramine, Promethazine) that cross the blood–brain barrier (BBB), are non-selective (blocking muscarinic, ̑-adrenergic, and serotonin receptors), and cause significant CNS sedation.

Second-Generation H₁ Blockers

Newer, more polar drugs (e.g., Loratadine, Cetirizine) that are more selective for peripheral H₁ receptors, do not cross the BBB, and have a longer duration of action ($12\text–24\,hrs$) with minimal sedation.

Least Sedating Second-Generation Agents

Desloratadine, Fexofenadine, and Loratadine.

Partially Sedating Second-Generation Agents

Cetirizine and Levocetirizine.

Prophylactic Use

The recommended administration of antihistamines before exposure to allergens, as they are more effective in prevention than in reversing induced symptoms.

Anticholinergic Side Effects

Effects due to muscarinic blockade by first-generation antihistamines, including dry mouth (xerostomia), blurred vision (due to cycloplegia), and urinary retention.

Anti-adrenergic Side Effects

Effects resulting from $\alpha$-blockade, such as hypotension, dizziness, or reflex tachycardia.

Cyproheptadine

An H₁ antihistamine that particularly acts on serotonin receptors, potentially altering appetite.

Azelastine

An ophthalmic antihistamine specifically used for the treatment of allergic conjunctivitis.

Motion Sickness Mechanism

Blocked by first-generation antihistamines (like Dimenhydrinate and Cyclizine) that inhibit $M_1$ muscarinic receptors in the vestibular and vomiting centers of the CNS.

Somnifacients (Sleep Aids)

First-generation agents such as Diphenhydramine and Doxylamine, used for short-term treatment of insomnia due to their strong sedative effects.

Pharmacokinetics (Half-life)

First-generation agents typically have a half-life of $4\text–6\,hrs$, while second-generation agents have a half-life of $12\text–24\,hrs$.

Additive Effect

The intensified drowsiness and sedation that occurs when first-generation antihistamines are used concurrently with other CNS depressants.

Headache

The most common adverse effect associated specifically with second-generation antihistamines.