Diphenhydramine: Uses, Dosage, Side Effects, and More
1. Drug Name
Generic Name: Diphenhydramine
Brand Name(s):
- Pakistan: Benadryl, Diphen
- India: Benadryl, Calmide
- United States: Benadryl, Nytol
- United Kingdom: Benadryl, Nytol
2. Pharmacological Class
Class: Antihistamine
Subclass: First-generation H1 antagonist
3. Mechanism of Action
Diphenhydramine works by blocking the action of histamine at H1 receptors. This prevents the symptoms of allergic reactions, such as itching, swelling, and rashes. It also has sedative properties, which make it useful as a sleep aid.
4. Indications
Primary Indications:
- Allergic reactions
- Insomnia
- Motion sickness
- Cold symptoms
Off-label Uses:
- Anxiety
- Parkinsonism symptoms
5. Dosage and Administration
Standard Dosage:
- Adults: 25-50 mg every 4-6 hours as needed, not to exceed 300 mg/day.
- Children: 12.5-25 mg every 4-6 hours as needed, not to exceed 150 mg/day.
Route of Administration: Oral (tablets, capsules, liquid), intravenous, and topical.
Special Populations:
- Pediatric: Use with caution in children under 6 years.
- Geriatric: Use with caution due to increased risk of sedation and anticholinergic effects.
6. Pharmacokinetics
Absorption: Well absorbed from the gastrointestinal tract.
Distribution: Widely distributed throughout the body, including the central nervous system.
Metabolism: Primarily metabolized in the liver.
Excretion: Excreted mainly in the urine.
Half-life: Approximately 4-8 hours.
7. Contraindications
Diphenhydramine is contraindicated in patients with a known hypersensitivity to diphenhydramine or any of its components. It should not be used in neonates or premature infants, or in breastfeeding women.
8. Warnings and Precautions
Use with caution in patients with narrow-angle glaucoma, asthma, hyperthyroidism, cardiovascular disease, or hypertension. Avoid alcohol and other CNS depressants while taking diphenhydramine.
9. Side Effects
Common Side Effects: Drowsiness, dizziness, dry mouth, blurred vision, constipation.
Serious Side Effects: Severe allergic reactions, hallucinations, seizures, severe drowsiness or fatigue.
10. Drug Interactions
Major Interactions: MAO inhibitors (prolonged anticholinergic effects), alcohol (increased CNS depression).
Moderate Interactions: Other CNS depressants (enhanced sedative effect), anticholinergic drugs (increased anticholinergic side effects).
Minor Interactions: Beta-blockers (increased risk of cardiovascular side effects).
11. Pregnancy and Lactation
Pregnancy Category: Category B - No proven risk in humans, but use only if clearly needed.
Breastfeeding Considerations: Diphenhydramine is excreted in breast milk and may cause side effects in the nursing infant. Use with caution.
12. Patient Counseling Information
Key Points for Patients:
- Take diphenhydramine exactly as directed. Do not exceed the recommended dose.
- Avoid driving or operating heavy machinery until you know how diphenhydramine affects you.
- Report any severe side effects or signs of allergic reaction to your doctor immediately.
Administration Tips:
- Take with a full glass of water. It can be taken with or without food.
- If using for motion sickness, take 30 minutes before travel.