💊 Diphenhydramine

First-Generation Antihistamine (H₁ Receptor Antagonist)

Generic Name	Diphenhydramine
Drug Class	First-Generation H₁ Antihistamine
Indications	Allergic conditions: rhinitis, urticaria, conjunctivitis Anaphylaxis (adjunct therapy with epinephrine) Motion sickness / vertigo Insomnia (short-term use)
Mechanism of Action	Blocks H₁ histamine receptors → reduces allergic symptoms (pruritus, edema, vasodilation); crosses blood-brain barrier → sedative effect
Available Forms & Strengths	• Tablet / Capsule: 25 mg, 50 mg • Oral solution / syrup: 12.5 mg/5 mL, 25 mg/5 mL • Injection: 10 mg/mL, 25 mg/mL
Dosage	Adults: 25–50 mg every 4–6 hours (max 300 mg/day) Children (6–12 years): 12.5–25 mg every 4–6 hours (max 150 mg/day) Children (2–6 years): 6.25 mg every 4–6 hours (max 37.5 mg/day)
Route	Oral, IM, IV
Common Side Effects	Drowsiness / sedation Dry mouth Blurred vision Constipation Urinary retention
Serious Adverse Effects	Respiratory depression (especially infants/elderly), severe arrhythmias (rare)
Contraindications	• Hypersensitivity to diphenhydramine • Newborns or premature infants (IV use) • Nursing mothers (caution)
Precautions	• Elderly (risk of sedation and falls) • Glaucoma, prostatic hypertrophy • Hepatic or renal impairment
Drug Interactions	• Alcohol / CNS depressants (additive sedation) • MAO inhibitors (↑ anticholinergic and sedative effects)
Pregnancy & Lactation	Pregnancy: Use if benefit outweighs risk Lactation: Caution; may cause sedation in infant
Storage	Store below 30°C, protect from moisture
References	BNF \| BNF for Children \| Goodman & Gilman

⚠️ Clinical note: Causes significant sedation; caution with driving or operating machinery. Monitor elderly for anticholinergic effects.