Overview
Diphenhydramine is a first-generation antihistamine used primarily for symptomatic relief of allergic conditions and as a sleep aid. It works by blocking histamine H1 receptors, reducing allergic symptoms and causing sedation. It is available over-the-counter in various formulations.
Mechanism of Action
Competitive antagonist at histamine H1 receptors, preventing histamine-mediated allergic responses. Also exhibits anticholinergic and sedative effects due to cross-reactivity with muscarinic receptors and central nervous system penetration.
Indications
- Temporary relief of upper respiratory allergy symptoms: runny nose, sneezing, itchy/watery eyes, itching of nose/throat
- Short-term treatment of insomnia
- Management of motion sickness
- Symptomatic relief of allergic skin reactions
Common Doses
- 12.5 mg/5 mL oral liquid
- 25 mg tablets/capsules
- 50 mg tablets/capsules
- 10 mg/mL injectable solution
Dosage
Adults: 25-50 mg every 4-6 hours as needed, not to exceed 300 mg daily. Children 6-11 years: 12.5-25 mg every 4-6 hours, not to exceed 150 mg daily. Children under 6 years: consult physician. For insomnia: 50 mg at bedtime.
Contraindications
- Hypersensitivity to diphenhydramine or similar antihistamines
- Neonates and premature infants
- Concurrent use with monoamine oxidase inhibitors (MAOIs)
- Acute asthma attacks
- Narrow-angle glaucoma
- Prostatic hypertrophy with urinary retention
Side Effects
- Common: Drowsiness, dry mouth, dizziness, headache, gastrointestinal upset
- Serious: Anaphylaxis, seizures, cardiac arrhythmias, severe hypotension, urinary retention
- Anticholinergic: Blurred vision, constipation, confusion (especially in elderly)
Interactions
- Alcohol and other CNS depressants: additive sedation
- MAOIs: hypertensive crisis risk
- Anticholinergic drugs: additive anticholinergic effects
- Drugs prolonging QT interval: increased arrhythmia risk
Counseling Points
- May cause significant drowsiness; avoid driving or operating machinery
- Avoid alcohol and other sedating medications
- Use caution in elderly due to increased risk of confusion and falls
- For children, use only under medical supervision for appropriate age group
- Do not use for prolonged periods without medical advice