UNISED PAEDIATRIC SYRUP
DIPHENHDRAMINE SODIUM CITRATE SYRUP
What it does
Diphenhydramine is an antihistamine used to relieve allergy symptoms and help with sleep.
Commonly used for: allergies (allergic rhinitis), hay fever, insomnia, motion sickness
Read more in plain English ↓Plain-language summary for general understanding - not medical advice. Always follow your pharmacist/doctor.
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Source: Pharmacy and Poisons Board · fetched 2026-01-28 21:32:47 · updated 2026-07-20 11:06:57
About this medicine
Diphenhydramine is an antihistamine used to relieve allergy symptoms and help with sleep.
What it treats
- allergies (allergic rhinitis)
- hay fever
- insomnia
- motion sickness
How it works
It blocks the effects of histamine, a substance in the body that causes allergic symptoms.
Who it's for
It is suitable for adults and children over a certain age, but always check with a healthcare provider.
AI-assisted summary grounded in BNF data - general information only, not medical advice. Always confirm with your pharmacist or doctor.
Clinical monograph: diphenhdramine
Diphenhydramine is an antihistamine that primarily blocks the action of histamine at H1 receptors. It is commonly used to relieve symptoms of allergy, hay fever, and the common cold, including rash, itching, watery eyes, itchy eyes/nose/throat, cough, runny nose, and sneezing. Additionally, it is used as a nighttime sleep aid due to its sedative properties.
Indications
- Allergic rhinitis
- Allergic conjunctivitis
- Urticaria
- Insomnia
- Motion sickness
- As an adjunct in anaphylactic reactions
Dosage
Children: Refer to BNF for Children for specific paediatric dosing guidelines.
Adults: Refer to BNF for specific adult dosing guidelines.
Mechanism of action
Diphenhydramine exerts its effects by competitively inhibiting H1 receptors, preventing the action of endogenous histamine. This action leads to decreased symptoms associated with allergic reactions. It also crosses the blood-brain barrier, leading to central nervous system effects, including sedation.
Pharmacodynamics
The primary pharmacodynamic effect of diphenhydramine is its antihistaminic action, which alleviates allergic symptoms. The sedative effects are due to its anticholinergic properties and ability to block H1 receptors in the CNS, leading to drowsiness. It may also have mild antitussive (cough-suppressant) properties.
Pharmacokinetics
Diphenhydramine is rapidly absorbed following oral administration, with peak plasma concentrations typically occurring within 1 to 2 hours. It has a volume of distribution of approximately 4 L/kg, and its metabolism primarily occurs in the liver via cytochrome P450 enzymes. The elimination half-life is approximately 4 to 8 hours in healthy adults, and it is excreted mainly in the urine, with some metabolites excreted in feces.
Contra-indications
- Hypersensitivity to diphenhydramine or any of its components
- Severe asthma attacks
- Narrow-angle glaucoma
- Prostate hypertrophy
Adverse effects
- Drowsiness
- Dizziness
- Dry mouth
- Blurred vision
- Constipation
- Urinary retention
Interactions
- Alcohol may enhance the sedative effect
- CNS depressants (e.g., benzodiazepines, opioids) can increase sedation
- MAO inhibitors may prolong and intensify the anticholinergic effects
- Anticholinergic agents may enhance the effects of diphenhydramine
Precautions
- Use with caution in elderly patients due to increased sensitivity to sedative effects
- Caution in patients with asthma or chronic obstructive pulmonary disease
- Avoid use in patients with a history of seizures
- Exercise caution in those with cardiovascular disease
Pregnancy
Diphenhydramine is classified as a Category B medication. Animal reproduction studies have not shown a risk to the fetus, but there are no adequate studies in pregnant women. It should be used only if clearly needed.
Breast-feeding
Diphenhydramine is excreted in breast milk. Caution is advised when administering to nursing mothers as it may cause sedation in the infant.
Storage
Store at room temperature, away from moisture and heat. Keep out of reach of children.
Formulations
- Oral tablets
- Oral liquid
- Injectable forms
- Topical creams
AI-synthesized from BNF references - general information only, not a substitute for professional medical advice or the current BNF. Verify doses with a pharmacist.