Phenothiazine Antihistamine·Allergy & Anaphylaxis

proMETHazine

Severe allergic reactions, severe nausea/vomiting, short-term insomnia

PBSANTIHISTAMINEPhenergan10 mg tabletS4+1

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At a glance

Max Dose

75 mg/day

Half-life

16–19 h

Caution: Anticholinergic

HIGH

High risk: IV Route

HIGH RISK

Cautions

Avoid / Cautions

Children < 2 years (Risk of fatal respiratory depression). Pregnancy Category C.

Where it stands

Rapid Summary

Overview

Highly sedating, first-generation antihistamine with phenothiazine neuroleptic properties. Used for extreme itching, severe nausea, and insomnia.

Clinical Focus

IV administration carries a severe 'black box' risk of tissue necrosis and gangrene. Prefer deep IM or PO.

Bottom Line

Promethazine is a potent first-generation sedating antihistamine useful for nausea and allergic reactions, but causes profound sedation and has significant anticholinergic burden especially in the elderly.

Formulation & Access

Brand Names

Phenergan

Oral Routes

Tablets (10 mg, 25 mg). Oral liquid.

Parenteral

Ampoule (50 mg/2 mL) for deep IM use. IV highly discouraged.

Prescribing & PBS

Pharmacy Medicine (S3 PO). Prescription (S4 Ampoules).

Indications & Use

Primary

PBSTGA

Severe allergic reactions, severe nausea/vomiting, short-term insomnia.

Clinical Place

Used when the sedating side effect is specifically desired (e.g., severe nocturnal itching preventing sleep).

Quick reference

Class & Role

Phenothiazine Antihistamine — Highly sedating, first-generation antihistamine with phenothiazine neuroleptic properties.

Route / Formulation

Tablets (10 mg, 25 mg). Oral liquid. (Phenergan)

Usual Dose & Max

PO 25 mg NOCTE. OR IM 25-50 mg STAT. Max 75 mg/day.

Key Indication Doses

Severe Allergic Reactions: PO 25 mg NOCTE. OR IM 25-50 mg STAT. Antiemetic / Motion Sickness: PO 25 mg PRN (TDS max). Insomnia (Short-term): PO 25-50 mg NOCTE.

Best Uses

Key Risks

Neurological: Profound sedation, hangover, confusion, delirium (especially in the elderly). Extrapyramidal side effects (EPSE) due to D2 blockade. Gastrointestinal: Severe dry mouth, constipation, urinary retention (Anticholinergic toxidrome). Tissue: Perivascular extravasation of IV Promethazine causes severe tissue necrosis and amputations.

Key Interactions

Additive CNS depression with opioids, benzos, and alcohol. Additive anticholinergic toxidrome with TCAs/antipsychotics.

Monitoring

Monitor respiratory rate if combined with opioids. Assess elderly for urinary retention and confusion. No specific routine laboratory tests required.

Clinical Pearl

Due to its pH, IV Promethazine causes devastating chemical burns if it leaks out of the vein, often resulting in finger/arm amputations. If you must use it parenterally, give it deep IM in a large muscle mass.

Access

Phenergan10 mg tablet

Brand: Phenergan
Formulation: Tablets (10 mg, 25 mg). Oral liquid.

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