Diphenhydramine — NZ Medication Guide
📋 Reviewed by a Registered Pharmacist NZ — This information is for general educational purposes only and does not replace professional medical advice. Always consult your doctor or pharmacist before starting, stopping, or changing any medication.
What is Diphenhydramine?
Diphenhydramine is a first-generation H1 antihistamine with significant sedative properties, used for allergic reactions and motion sickness. This medication is available in some OTC combination products; injection funded by Pharmac for acute allergic reactions.
What is Diphenhydramine Used For?
Used for acute allergic reactions and urticaria, motion sickness, and short-term insomnia. IV/IM form used as adjunct in anaphylaxis.
How Does Diphenhydramine Work?
Blocks H1 histamine receptors with significant CNS penetration, causing sedation alongside antiallergic effects. Also has strong anticholinergic properties.
How to Take Diphenhydramine
25–50 mg every 4–6 hours for allergy. 25–50 mg at bedtime for sleep. Avoid in elderly patients. Do not drive.
Common Side Effects of Diphenhydramine
- Significant sedation (pronounced — do not drive)
- Dry mouth
- Blurred vision
- Urinary retention
- Constipation
Serious Side Effects — Seek Medical Attention
Contact your doctor or call 111 immediately if you experience any of the following:
- Falls and injury in elderly (anticholinergic and sedative effects — Beers Criteria inappropriate)
- Urinary retention in BPH
- Anticholinergic toxidrome at high doses
- Paradoxical excitation in children
Drug Interactions
Diphenhydramine may interact with other medicines. Always inform your doctor and pharmacist of all medications you are taking. Key interactions include:
- CNS depressants and alcohol (severe additive sedation)
- MAOIs (contraindicated)
- Other anticholinergic medicines (additive)
New Zealand Prescribing Information
Diphenhydramine injection is funded by Pharmac for acute allergic reactions. Second-generation antihistamines (cetirizine, loratadine, fexofenadine) are strongly preferred for routine allergy management. Diphenhydramine should be avoided in elderly patients due to high anticholinergic burden — listed as potentially inappropriate in the Beers Criteria.
Frequently Asked Questions
Is diphenhydramine safe for elderly patients?
No — diphenhydramine is listed as potentially inappropriate for elderly patients (Beers Criteria) due to its high anticholinergic and sedative profile, which significantly increases fall risk, confusion, and urinary retention in this age group. Second-generation antihistamines are preferred.
Can diphenhydramine be used as a regular sleep aid?
Tolerance develops rapidly (within days), and the significant next-day cognitive impairment makes it unsuitable for regular use. CBT-I is the recommended first-line treatment for insomnia.
References & Further Information
The following New Zealand and international resources were used to inform this page:
- New Zealand Formulary — Diphenhydramine
- American Geriatrics Society Beers Criteria
- Pharmac — Funded Medicines Schedule
⚠️ Disclaimer: This page is for educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your doctor or a qualified health provider. In New Zealand, medication availability and funding may vary — check with Pharmac or your pharmacist for current information.
Reviewed by a Registered Pharmacist NZ