1. Introduction
Diphenhydramine is a first-generation antihistamine used to treat allergies, insomnia, motion sickness, and cold symptoms. It works by blocking histamine H1 receptors, alleviating symptoms such as itching, sneezing, and runny nose. Additionally, it has sedative and anticholinergic properties, making it useful for sleep disorders and nausea.
2. Indications
- Approved Indications:
- Allergic rhinitis and conjunctivitis.
- Urticaria (hives) and other allergic skin reactions.
- Anaphylaxis (adjunct to epinephrine).
- Insomnia (short-term use).
- Motion sickness.
- Common cold symptoms (e.g., runny nose, sneezing).
- Off-Label Uses:
- Nausea and vomiting.
- Management of extrapyramidal symptoms (drug-induced movement disorders).
3. Dosage
- Adults:
- Allergic conditions: 25–50 mg every 4–6 hours as needed (maximum 300 mg/day).
- Insomnia: 50 mg taken 30 minutes before bedtime.
- Motion sickness: 25–50 mg 30 minutes before travel and then every 4–6 hours as needed.
- Children (6–12 years):
- Allergic conditions: 12.5–25 mg every 4–6 hours (maximum 150 mg/day).
- Motion sickness: 12.5–25 mg 30 minutes before travel.
- Elderly:
- Use the lowest effective dose due to increased risk of sedation and anticholinergic side effects.
4. Administration
- Take orally with or without food.
- For motion sickness, administer 30 minutes before travel.
- Use only as directed for short-term relief of symptoms.
5. Forms Available
- Tablets: 25 mg, 50 mg.
- Liquid oral solution: 12.5 mg/5 mL.
- Capsules: 25 mg, 50 mg.
- Topical cream: 2%.
- Injectable solution: 50 mg/mL (hospital use).
6. Side Effects
6.1. Common Side Effects
- Drowsiness.
- Dry mouth, nose, or throat.
- Dizziness.
6.2. Less Common Side Effects
- Constipation or urinary retention.
- Blurred vision.
- Restlessness or nervousness (especially in children).
6.3. Rare but Serious Side Effects
- Severe allergic reactions (e.g., rash, swelling, difficulty breathing).
- Confusion or hallucinations, particularly in elderly patients.
- Tachycardia or palpitations.
7. Warnings
- CNS Depression: May impair mental alertness and motor coordination. Avoid activities requiring full alertness, such as driving.
- Overdose Risk: Overdose can cause severe drowsiness, seizures, or hallucinations, and is particularly dangerous in children.
- Anticholinergic Effects: Use cautiously in patients with conditions such as glaucoma, prostatic hypertrophy, or urinary retention.
8. Precautions
- Avoid use in children under 6 years without medical advice.
- Avoid concurrent use with other CNS depressants, including alcohol.
- Not recommended for chronic insomnia due to risk of dependence and tolerance.
9. Interactions
- Drug-Drug Interactions:
- CNS depressants (e.g., alcohol, benzodiazepines): Increased sedation.
- Monoamine oxidase inhibitors (MAOIs): Prolonged anticholinergic effects.
- Anticholinergics (e.g., atropine): Increased risk of side effects like dry mouth or constipation.
- Drug-Food Interactions:
- Avoid alcohol, as it may enhance sedative effects.
10. Monitoring Parameters
- Relief of allergic symptoms or insomnia.
- Signs of excessive sedation or anticholinergic side effects.
- In long-term use, monitor for tolerance or dependence.
11. Use in Children
- Safe for short-term use in children over 6 years.
- Use cautiously, as paradoxical excitation (e.g., restlessness or agitation) may occur.
12. Use in Pregnancy
- Category B: Generally considered safe for short-term use during pregnancy, but consult a healthcare provider.
- Avoid prolonged use due to potential sedation of the foetus.
13. Use in Elderly
- Use with caution due to increased sensitivity to sedative and anticholinergic effects, such as confusion, dry mouth, and urinary retention.
14. Use in Kidney Disease
- No specific dose adjustment required, but monitor for side effects as clearance may be reduced in severe renal impairment.
15. Use in Liver Disease
- Use cautiously in patients with significant hepatic impairment, as metabolism may be reduced.
16. Patient Counselling Points
- Take diphenhydramine only as needed and avoid exceeding the recommended dose.
- Avoid driving or operating heavy machinery after taking this medication due to its sedative effects.
- Avoid alcohol and other sedatives while on this medication.
- Use for allergies or colds should not exceed 7–10 days without consulting a doctor.
- Report severe side effects such as confusion, fast heartbeat, or hallucinations immediately.
17. Table of Brand Names, Manufacturers, and Prices
Brand Name | Manufacturer | Price (USD) |
---|---|---|
Benadryl | Johnson & Johnson | $5–10 (24 tablets, 25 mg) |
Generic Diphenhydramine | Various | $3–8 (24 tablets, 25 mg) |
Nytol | Omega Pharma | $6–12 (20 tablets, 50 mg) |
18. FAQs (15 Questions)
- What is diphenhydramine used for?
Diphenhydramine is used to treat allergies, insomnia, motion sickness, and cold symptoms. - How does diphenhydramine work?
It blocks histamine H1 receptors, reducing allergic symptoms and causing sedation. - Can diphenhydramine cause drowsiness?
Yes, drowsiness is one of the most common side effects. - Is diphenhydramine safe for children?
It can be used in children over 6 years under medical guidance. Avoid in children under 6 without a doctor’s advice. - Can diphenhydramine be used during pregnancy?
It is generally considered safe for short-term use but should be used under medical supervision. - What are the common side effects of diphenhydramine?
Drowsiness, dry mouth, and dizziness are common side effects. - Can diphenhydramine be taken for insomnia?
Yes, it is effective for short-term management of insomnia. - How long does diphenhydramine take to work?
It usually begins working within 30–60 minutes of administration. - Can diphenhydramine cause confusion?
Yes, especially in elderly patients, confusion or mental fog may occur. - Can diphenhydramine be addictive?
While not physically addictive, it may cause psychological dependence if used long-term for insomnia. - What happens if I take too much diphenhydramine?
Overdose can cause severe drowsiness, confusion, hallucinations, and even seizures. Seek immediate medical attention. - Can diphenhydramine be taken with alcohol?
No, alcohol enhances sedation and increases the risk of side effects. - Is diphenhydramine available over the counter?
Yes, it is widely available without a prescription. - How should diphenhydramine be stored?
Store at room temperature, away from light and moisture. - Can I use diphenhydramine long-term?
Long-term use is not recommended due to the risk of tolerance and side effects.