Promethazine
A strongly sedating phenothiazine antihistamine for allergies, nausea and motion sickness, and for short-term sedation.
Drug card
- How it works
- Blocking H1 receptors in body and brain, so they also sedate
- Used for
- Allergic rhinitis, conjunctivitis and hives
- Nausea and vomiting after anesthesia and surgery
- Motion sickness
- Sedation before and after surgery
- Known for
- Marked drowsiness and sedation
- Fatal respiratory depression in young children
Study it as a flashcard, and it comes back before you forget it.
Mechanism of action
How does promethazine work?
A phenothiazine that blocks H1 receptors throughout the body; it enters the brain in large amounts, where the same block causes marked sedation. It blocks muscarinic receptors and, weakly (about a tenth as strongly as chlorpromazine), dopamine receptors; these actions add anticholinergic and antiemetic effects.
Like other first-generation antihistamines: First-generation antihistamines competitively and reversibly block histamine H1 receptors, so histamine released in allergy can no longer make small vessels leak and swell, cause itch or contract smooth muscle. They enter the brain readily and block its H1 receptors, which causes sedation, and they also block muscarinic receptors, giving anticholinergic effects such as dry mouth and blurred vision.
Uses
What is promethazine used for?
- Allergic rhinitis, conjunctivitis and hives
- Nausea and vomiting after anesthesia and surgery
- Motion sickness
- Sedation before and after surgery
- Short-term insomnia in adults
- Anaphylaxis, as an add-on after epinephrine
Side effects
Side effects of promethazine
- Marked drowsiness and sedationKnown for
- Fatal respiratory depression in young childrenKnown forSerious
- Dry mouth, blurred vision, constipation
- Difficulty passing urine
- Restlessness, nightmares and excitement in children
- Dystonia and other extrapyramidal effects
- Neuroleptic malignant syndromeSerious
- QT prolongation and torsade de pointesSerious
- Sensitivity to sunlight
- Agranulocytosis and other blood disordersSerious
- Cholestatic jaundiceSerious
- Gangrene if the injection damages tissueSerious
Known for marks what an exam expects you to know it for.
Watch for
Cautions and what sets it apart
- Boxed warning: fatal respiratory depression in children under 2; UK labels exclude children under 6.
- Boxed warning (injection): severe tissue injury, including gangrene; it is never given into an artery or under the skin.
- Alcohol, opioids, sedatives and other CNS depressants add to its sedation and breathing suppression.
- Avoid with MAO inhibitors and QT-prolonging drugs; care in glaucoma, prostate enlargement, epilepsy and COPD.
Good to know: Its weak dopamine block, about a tenth of chlorpromazine's, comes with rare extrapyramidal effects and NMS, as with other phenothiazines.
Remember it
A way to remember it
Old antihistamines get into your head.
First-generation drugs enter the brain, so they cause drowsiness (and in children sometimes excitement), and they block muscarinic receptors too, drying the mouth and holding back urine; the newer ones mostly stay out.
Written for biofideX
Its class
First-generation antihistamines
Other first-generation antihistamines with a card:
Easily mixed up with second-generation antihistamines, H2 receptor antagonists and antimuscarinics.
Sources
Checked against
Written for biofideX in plain words, after checking these references:
- Promethazine hydrochloride tablets: label (DailyMed)
- Promethazine hydrochloride injection: label (DailyMed)
- Promethazine tablets: SmPC (emc)
- Promethazine hydrochloride (NCI Thesaurus)
- Promethazine (NHS)
For learning pharmacology, not for treatment decisions: no doses are given, and a prescriber, pharmacist or the product's own label is the word on any one patient. Spotted a mistake? Tell us and we'll correct it.
Lungs and allergy
Promethazine is one of 16 drugs practiced together here: their classes, how they work, what they treat and what they're known for.
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