Antihistamines

Key Points

• Antihistamines block H1 histamine receptors to reduce allergic symptoms.
• First-generation agents (diphenhydramine) cross the blood-brain barrier and cause sedation.
• Second-generation agents (loratadine, cetirizine) have reduced CNS penetration and less sedation.
• Used for allergic rhinitis, urticaria, pruritus, and as adjuncts in anaphylaxis management.

Mechanism of Action

Antihistamines competitively block H1 histamine receptors on target cells, preventing histamine from triggering the allergic response cascade. This reduces vasodilation, increased capillary permeability, bronchoconstriction, and pruritus associated with histamine release.

Classification

• First generation (diphenhydramine, chlorpheniramine, hydroxyzine): Cross the blood-brain barrier; cause significant sedation and anticholinergic effects.
• Second generation (loratadine, cetirizine, fexofenadine): Minimal CNS penetration; less sedation and fewer anticholinergic effects.

Indications

• Allergic rhinitis (seasonal and perennial).
• Urticaria (hives) and pruritus.
• Adjunct therapy in allergic reactions and anaphylaxis.
• Motion sickness and nausea (first-generation agents).
• Sleep aid (diphenhydramine — due to sedation effect).

Nursing Considerations

• Assess allergy history and symptom severity before selecting agent.
• First-generation agents increase fall risk in elderly due to sedation and anticholinergic effects; use second-generation agents when possible.
• Monitor for anticholinergic effects with first-generation agents: dry mouth, urinary retention, constipation, blurred vision.
• Avoid concurrent use with CNS depressants (alcohol, opioids, benzodiazepines) due to additive sedation.
• Diphenhydramine is on the Beers Criteria list for potentially inappropriate medications in older adults.

Side Effects and Adverse Effects

• First generation: Sedation, drowsiness, dry mouth, blurred vision, urinary retention, constipation, thickened bronchial secretions.
• Second generation: Headache, mild drowsiness (less common), GI upset.
• Serious: Paradoxical excitation in children; severe anticholinergic toxicity with overdose.

Health Teaching

• First-generation antihistamines may cause significant drowsiness; avoid driving or operating machinery.
• Avoid alcohol and other CNS depressants during therapy.
• Take second-generation agents for daytime allergy relief to minimize sedation.
• Report difficulty urinating, excessive drowsiness, or rapid heartbeat.

Related Concepts

• anaphylaxis - Adjunct therapy context for acute allergic reactions.
• corticosteroids - Often used in combination for allergic inflammation control.
• infection-control - Immune response and allergy context.

Self-Check

1. Why are second-generation antihistamines preferred for daytime allergy management?
2. What anticholinergic side effects should be monitored with first-generation antihistamines?
3. Why is diphenhydramine on the Beers Criteria list for older adults?