H1 Antihistamine (1st Gen)·Allergy & Anaphylaxis

Diphenhydramine

Short-term insomnia, allergic rhinitis, acute urticaria/allergic reactions, motion sickness

ANTIHISTAMINEBenadryl (Sleep/Allergy formulations), Snuza25 mg tabletS3TGA

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At a glance

Max Dose

Sleep: 50 mg nocte

Half-life

2-8 h

Caution: Anticholinergic

HIGH RISK

Caution: Sedation

PROFOUND

Cautions

Avoid / Cautions

Narrow-angle glaucoma, severe BPH, acute asthma attack (dries up respiratory secretions into thick mucus plugs). Pregnancy Category A (Safe, but newer 2nd gen agents like Loratadine are preferred).

Where it stands

Rapid Summary

Overview

First-generation H1 antihistamine. Highly lipophilic, easily crossing the blood-brain barrier. Causes profound sedation and intense anticholinergic effects.

Clinical Focus

Extremely dangerous in the frail elderly due to massive risk of delirium, falls, and acute urinary retention.

Bottom Line

Diphenhydramine is a first-generation antihistamine for allergic reactions and short-term insomnia, but has potent anticholinergic effects making it dangerous in elderly and causes significant next-day sedation.

Formulation & Access

Brand Names

Benadryl (Sleep/Allergy formulations), Snuza

Oral Routes

Capsules/Tablets (25 mg, 50 mg). Oral liquid.

Parenteral

Ampoules for IV / IM use (Mostly used in EDs/OTs).

Prescribing & PBS

Pharmacy Medicine (S3).

Indications & Use

Primary

TGA

Short-term insomnia, allergic rhinitis, acute urticaria/allergic reactions, motion sickness.

Off-Label

OFF

Acute dystonic reactions (muscle spasms caused by Haldol/Maxolon), EPS management.

Clinical Place

Primarily used as an OTC sleep aid or a rapid rescue agent for dystonia/anaphylaxis in the ED.

Quick reference

Class & Role

H1 Antihistamine (1st Gen) — First-generation H1 antihistamine.

Route / Formulation

Capsules/Tablets (25 mg, 50 mg). Oral liquid. (Benadryl (Sleep/Allergy formulations), Snuza)

Usual Dose & Max

PO 25-50 mg NOCTE (taken 30 mins before bed). Max 300 mg/day (Usually given as 25-50 mg per dose).

Key Indication Doses

Insomnia (Short-term): PO 25-50 mg NOCTE (taken 30 mins before bed). Severe Allergic Reactions: PO / IV / IM 25-50 mg q6-8h PRN. Acute Dystonia (Antipsychotic-induced): IV / IM 25-50 mg STAT (if Benzatropine is unavailable).

Best Uses

Key Risks

Neurological: Toxic delirium, confusion, hallucinations (especially in elderly or overdose). HIGH — Next-day hangover, profound sedation, impaired motor skills. Gastrointestinal: Severe dry mouth, constipation. Genitourinary: Acute urinary retention (precipitates retention in men with BPH).

Key Interactions

Additive CNS depression with Opioids, Benzos, and Alcohol (frequent cause of accidental overdose). TCAs, Antipsychotics, Oxybutynin (Massive additive anticholinergic toxidrome).

Monitoring

Monitor bladder output in older patients. Ensure a clear path to the bathroom to prevent falls. No specific routine laboratory tests required.

Clinical Pearl

If a young patient is given IV Metoclopramide (Maxolon) for nausea and suddenly develops a locked jaw or their neck violently spasms backward (oculogyric crisis), they are having an acute dystonic reaction. 50mg of IV/IM Diphenhydramine (or Benzatropine) will cure the spasm in 60 seconds by resetting the dopamine-acetylcholine balance in the brain.

Access

Benadryl (Sleep/Allergy formulations), Snuza25 mg tablet

Brand: Benadryl (Sleep/Allergy formulations), Snuza
Formulation: Capsules/Tablets (25 mg, 50 mg). Oral liquid.

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