Antihistamines

Antihistamines

Drug monograph · NCLEX study reference
Trade names	Benadryl, Zyrtec
Therapeutic class	Antihistamine
Pharmacologic class	H1 receptor antagonists (sedating first generation, nonsedating second generation)
Onset / peak / duration	First generation onset within 30 minutes, duration 4 to 6 hours; second generation once daily.
Half-life / level	Varies; no routine level.
Routes	PO (oral), IV, IM
High-alert (ISMP)	No
Black box warning	None
Antidote / reversal	Supportive care.
Pregnancy / lactation	Some used when needed.

Nursing pharmacology study reference (NCLEX-style monograph). Numeric values are standard teaching ranges for study and must be verified against current manufacturer labeling before clinical use. This is educational content, not prescribing guidance.

Antihistamines (brand names Benadryl, Zyrtec) — Antihistamine; H1 receptor antagonists (sedating first generation, nonsedating second generation).

Identification

• Therapeutic class: Antihistamine.
• Pharmacologic class: H1 receptor antagonists (sedating first generation, nonsedating second generation).

Pharmacology

• Mechanism of action: Block H1 histamine receptors to reduce allergy symptoms; first generation crosses into the brain (sedation).
• Onset / peak / duration: First generation onset within 30 minutes, duration 4 to 6 hours; second generation once daily.
• Half-life / therapeutic level: Varies; no routine level.

Clinical use

• Indications: Allergic rhinitis, urticaria, allergic reactions; diphenhydramine also for sleep, motion sickness, and as an anaphylaxis adjunct and for dystonia.
• Usual dose, route, frequency: Diphenhydramine PO, IM, or IV; second generation PO once daily.
• Maximum dose / adjustments: Caution with first generation in elderly (anticholinergic and fall risk).

Safety

• Contraindications: First generation in narrow-angle glaucoma, urinary retention, and neonates.
• Black box warning: None.
• Interactions: CNS depressants and alcohol (sedation, first generation), other anticholinergics.
• Pregnancy / lactation: Some used when needed.
• High-alert: No.

Adverse effects

• Common side effects: Sedation, dry mouth, and dizziness (first generation); minimal sedation (second generation).
• Serious effects to report: Anticholinergic toxicity, severe sedation, paradoxical excitation in children.
• Antidote / reversal: Supportive care.

Nursing process

• Assessment before administration: Allergy symptoms, sedation risk, glaucoma and urinary history.
• Interventions during therapy: Prefer second generation for daytime; caution with driving (first generation).
• Monitor: Symptom relief, sedation.
• Evaluation / expected outcome: Reduced allergy symptoms.

Patient teaching

• Patient teaching: First-generation antihistamines cause drowsiness (avoid driving and alcohol); second-generation cause little drowsiness.
• Notify provider if: Severe drowsiness, urinary difficulty, confusion.
• Administration tips: Second generation for daily nonsedating use; first generation short term.