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Antihistamines

4 min read

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.
  • Third-generation agents (fexofenadine, desloratadine) are designed to preserve efficacy with lower CNS adverse-effect burden.
  • Used for allergic rhinitis, urticaria, pruritus, and as adjuncts in anaphylaxis management.
  • In ear-symptom pathways, antihistamines can reduce upper-airway congestion but require anticholinergic and sedation risk screening.

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.
  • Third generation (fexofenadine, desloratadine): Peripheral H1 selectivity with low CNS penetration; generally preferred when sedation avoidance is important.

Indications

  • Allergic rhinitis (seasonal and perennial).
  • Urticaria (hives) and pruritus.
  • Temporary relief of selected common-cold symptoms (for example rhinorrhea and sneezing) when product/age criteria are met.
  • 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.
  • Do not use antihistamine products in children younger than 2 years without a prescriber order.
  • First-generation onset is commonly about 30-60 minutes with relief around 4-6 hours, so timing and repeat-dose education matter for safe self-administration.
  • For allergic-rhinitis pathways, dosing at symptom onset is generally more effective than delayed use after symptoms escalate.
  • In urticaria pathways, antihistamines are commonly first-line symptom therapy for itching and swelling; escalate if airway symptoms or progressive angioedema develops.
  • In atopic-dermatitis itch pathways, first-generation oral antihistamines are often scheduled at bedtime for sedative support; daytime dosing can cause unwanted drowsiness.
  • 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.
  • Use extra caution in clients with glaucoma, urinary-retention risk, uncontrolled hypertension, or dysrhythmia susceptibility when self-treating ear/allergy congestion symptoms.
  • In antihistamine-decongestant combination products, monitor blood pressure trends and avoid prolonged self-use in clients with hypertension risk.
  • 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.
  • In older adults, avoid first-generation agents when possible because anticholinergic burden and slower clearance increase cognitive and fall risk.

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 including nausea/vomiting, fatigue, and possible dysmenorrhea.
  • Third generation: Fatigue, dizziness, nausea, and insomnia may still occur even with lower sedative burden.
  • 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.
  • Use frequent oral hygiene strategies (for example hydration, oral care, sugar-free lozenges when appropriate) to reduce dry-mouth discomfort.
  • Report difficulty urinating, excessive drowsiness, or rapid heartbeat.
  • anaphylaxis - Adjunct therapy context for acute allergic reactions.
  • corticosteroids - Often used in combination for allergic inflammation control.
  • infection-control - Immune response and allergy context.
  • Otic Cerumenolytics - Ear-focused antihistamine/decongestant and cerumenolytic safety 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?

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