Benadryl
Benadryl
Generic Name
Benadryl
Mechanism
- H1‑receptor blockade: competitively inhibits peripheral and central H1 receptors, dampening histamine‑mediated symptoms (urticaria, pruritus, rhinitis, bronchoconstriction).
- Anticholinergic activity: inhibits muscarinic receptors in the CNS and periphery, contributing to sedation, dry mouth, constipation, and tachycardia.
- Mild antiserotonergic effect: may influence nausea and motion sickness.
*Result: transient relief of allergic inflammation and central sedation.*
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Pharmacokinetics
| Parameter | Typical values (adult) |
| Absorption | Oral bioavailability ~ 60–70 %; peak plasma 30–60 min |
| Distribution | Widely distributed; crosses BBB → CNS side effects |
| Protein binding | ~65% (mainly albumin) |
| Metabolism | Hepatic N‑demethylation → active metabolite (promethazine‑like), then glucuronidation |
| Elimination | Renal (≈50 %) and fecal; CL ≈ 0.7 L·min⁻¹ |
| Half‑life | 3–4 h (extended‑release ~ 8–12 h) |
| Drug interactions | Potentiated sedation with CNS depressants, CYP2D6 inhibitors ↑ levels |
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Indications
- Allergic reactions: urticaria, hives, mild anaphylaxis (with epinephrine)
- Allergic rhinitis / hay fever
- Dermatologic: pruritus, dermatitis, insect bites
- Motion sickness & vertigo (off‑label)
- Insomnia: short‑term sleep aid (sedative property)
- Adjunct in asthma (anti‑inflammatory component, limited)
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Contraindications
| Category | Details |
| Contraindications | |
| • Allergy to diphenhydramine or other alkylamine antihistamines. | |
| • Severe hepatic impairment (reduced metabolism). | |
| Warnings | |
| • Anticholinergic toxicity (elderly, Parkinson’s, prostatic hypertrophy). | |
| • Risk of sedation, especially when combined with alcohol or CNS depressants. | |
| • Cardiovascular: potential for tachycardia, arrhythmias (especially overdose). | |
| • Not for chronic insomnia; use only short‑term. |
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Dosing
| Population | Adult | Pediatric (≥6 y) | Pediatric (2–6 y) |
| Oral PO | 25–50 mg q4–6 h; max 400 mg/day | 1 mg/kg (max 50 mg) q4–6 h | 0.5 mg/kg (max 25 mg) q4–6 h |
| IV/IM | 12.5–25 mg q2–3 h (max 100 mg/day) | 0.5–1 mg/kg q2–3 h (max 50 mg/day) | – |
| Extended‑release (ER) | 60–120 mg q8 h | – | – |
| Topical | 0.5–1% cream, 1–2 tsp, q4–6 h | 0.5–1% cream, 1 tsp, q4–6 h | 0.5% cream, ½ tsp, q4–6 h |
• Take with food if GI upset occurs.
• Avoid alcohol; combine cautiously with other sedatives.
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Adverse Effects
Common (↑ 5 %):
• Sedation, drowsiness, fatigue
• Dry mouth, blurred vision, constipation
• Tachycardia, tremor
• Urinary retention (prostatic obstruction)
Serious (↑ < 1 %):
• Severe hypotension, arrhythmias
• Extrapyramidal symptoms, akathisia
• Anticholinergic crisis (confusion, hallucinations, seizures)
• Respiratory depression (with overdose)
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Monitoring
- Vital signs: blood pressure, heart rate (watch for tachycardia, arrhythmias).
- Sedation level: especially in elderly or opioid users.
- Signs of anticholinergic toxicity: confusion, agitation, urinary retention.
- Elderly patients: dose‑adjust based on renal/hepatic function and cognitive status.
- Children: monitor for over‑sedation and respiratory suppression.
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Clinical Pearls
- First‑line for mild allergic urticaria: Diphenhydramine rapidly resolves hives; consider with antihistamine‑resistant cases.
- Sedation as a therapeutic advantage: Utilize in patients needing brief sleep aid but avoid long‑term sleep therapy.
- Avoid in geriatric acute delirium: Anticholinergic burden increases fall risk; switch to second‑generation antihistamines if needed.
- Double‑dose caution: 25 mg is the minimum effective dose; exceeding 50 mg increases anticholinergic side effects without added benefit.
- Drug interaction hot‑spot: Combine carefully with benzodiazepines, opioids, or alcohol – risk of cumulative CNS depression.
- Overdose management: Defer to standard H1‑antagonist protocols; supportive airway, cardiac monitoring; consider benzylpenicillin for histamine blockade if severe anaphylaxis.
- Dermatology tip: For localized itching, use 0.5–1 % diphenhydramine cream; systemic dosing reserved for generalized symptoms.
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• *For up‑to‑date dosing guidance, consult FDA prescribing information, UpToDate, or local clinical guidelines. Always tailor therapy to patient comorbidities and concurrent medications.*