Mechanism of action
Promethazine is a competitive antagonist at histamine H₁ receptors in the central nervous system and periphery, blocking the effects of histamine on smooth muscle, vascular endothelium, and sensory nerves. This accounts for its antiallergic and antipruritic effects.
In addition to H₁ blockade, promethazine possesses substantial anticholinergic (muscarinic) activity at M₁ receptors, which contributes to its antiemetic and sedative properties by blocking vestibular input and reducing motion sickness. It also blocks dopamine D₂ receptors in the chemoreceptor trigger zone, providing additional antiemetic action.
Promethazine exhibits alpha-adrenergic blocking effects, which can cause orthostatic hypotension, and has local anesthetic properties that contribute to tissue irritation when administered parenterally. Its sedative effects are mediated through central H₁ blockade and anticholinergic activity in the reticular activating system.
- Black box warning: contraindicated in children under 2 years due to fatal respiratory depression risk
- Black box warning for IV use: severe tissue injury including gangrene with extravasation or intra-arterial injection
- Phenothiazine with H₁ blockade plus anticholinergic, antidopaminergic, and alpha-blocking properties
- Paradoxical hypotension with epinephrine due to alpha-adrenergic blockade—use norepinephrine instead
- Can cause extrapyramidal symptoms (dystonia, akathisia) due to D₂ receptor blockade
- Potentiates opioid-induced respiratory depression; reduce doses when used together
- Lowers seizure threshold; use with caution in patients with epilepsy
- Avoid in elderly (Beers Criteria) due to strong anticholinergic effects and fall risk
Indications
Promethazine is approved for multiple indications across allergy, nausea, and sedation:
Allergic conditions
- Allergic rhinitis (seasonal and perennial)
- Urticaria and pruritus
- Allergic conjunctivitis
- Mild allergic reactions to blood or plasma
Nausea and vomiting
- Prevention and treatment of nausea and vomiting from various causes
- Motion sickness (prophylaxis and treatment)
- Postoperative nausea and vomiting
Sedation
- Preoperative and postoperative sedation
- Adjunct to analgesics for sedation and relief of apprehension
- Nighttime sedation (short-term use)
Promethazine is often combined with opioid analgesics to potentiate their effects and provide sedation, though this combination increases respiratory depression risk.
Pharmacokinetics
Promethazine is well absorbed after oral, rectal, and intramuscular administration, with onset of action within 20 minutes. Peak plasma concentrations occur 2-3 hours after oral dosing. It is highly lipophilic and crosses the blood-brain barrier readily, accounting for pronounced CNS effects.
The drug undergoes extensive first-pass hepatic metabolism via sulfoxidation and demethylation by cytochrome P450 enzymes, primarily CYP2D6. Metabolites are excreted in urine and feces. The elimination half-life ranges from 10 to 19 hours, allowing once or twice-daily dosing.
Promethazine is highly protein-bound (93%) and has a large volume of distribution. It crosses the placenta and is present in breast milk.
Adverse effects
Promethazine carries significant adverse effect risks due to its multiple receptor activities:
Central nervous system
- Sedation and drowsiness (most common, dose-dependent)
- Confusion, disorientation, dizziness
- Paradoxical excitation in children
- Extrapyramidal symptoms (dystonia, akathisia) due to dopamine blockade
- Lowered seizure threshold
- Neuroleptic malignant syndrome (rare)
Anticholinergic effects
- Dry mouth, blurred vision, urinary retention
- Constipation
- Tachycardia
- Anticholinergic toxicity in overdose
Cardiovascular
- Orthostatic hypotension (alpha blockade)
- QT prolongation and torsades de pointes (rare)
- Bradycardia or tachycardia
Respiratory
- Respiratory depression (especially in children under 2 years — black box warning)
- Thickening of bronchial secretions
Local reactions
- Severe tissue injury with IV administration: chemical irritation, thrombophlebitis, tissue necrosis, and gangrene if extravasation occurs
- Pain and induration at IM injection sites
Hematologic
- Leukopenia, agranulocytosis (rare)
- Thrombocytopenia
Other
- Photosensitivity
- Cholestatic jaundice (rare)
- Allergic reactions including angioedema
Contraindications and warnings
Black box warning
Promethazine carries a black box warning for use in children under 2 years of age due to fatal respiratory depression. It should not be used in this age group. Caution is advised in older children due to risk of respiratory depression and extrapyramidal symptoms.
Additionally, promethazine injection has a black box warning for severe tissue injury, including gangrene requiring amputation. Intravenous administration must be through a functioning IV line with careful monitoring; subcutaneous and intra-arterial routes are contraindicated.
Absolute contraindications
- Children under 2 years of age
- Comatose states
- Concomitant use with MAO inhibitors (within 14 days)
- Known hypersensitivity to phenothiazines
- Lower respiratory tract symptoms (including asthma) in children due to risk of respiratory depression
Warnings and precautions
- Elderly patients: Increased risk of falls, confusion, anticholinergic toxicity
- Respiratory disease: Can suppress cough reflex and thicken secretions
- Cardiovascular disease: Risk of hypotension and QT prolongation
- Seizure disorders: Lowers seizure threshold
- Narrow-angle glaucoma, prostatic hypertrophy, stenosing peptic ulcer, bladder neck obstruction: Anticholinergic effects may worsen these conditions
- Bone marrow suppression: Monitor blood counts with prolonged use
- Pregnancy category C: Use only if benefit outweighs risk; may cause platelet dysfunction in neonates if used near term
- Breastfeeding: Excreted in breast milk; may cause sedation or paradoxical excitation in infants
Drug interactions
Promethazine has numerous clinically significant drug interactions:
CNS depressants
- Opioids, benzodiazepines, barbiturates, alcohol: Additive sedation and respiratory depression; reduce doses of both agents
- General anesthetics: Enhanced CNS and respiratory depression
Anticholinergic agents
- Tricyclic antidepressants, antihistamines, antiparkinsonian drugs: Additive anticholinergic effects (dry mouth, urinary retention, confusion, ileus)
MAO inhibitors
- Contraindicated: Can prolong and intensify anticholinergic effects; allow 14-day washout period
Medications affecting cardiac conduction
- QT-prolonging drugs (antiarrhythmics, macrolides, fluoroquinolones, antipsychotics): Increased risk of torsades de pointes
Epinephrine
- Promethazine's alpha-blocking effects can reverse epinephrine's pressor effects, leading to paradoxical hypotension; use norepinephrine if vasopressor needed
Other interactions
- CYP2D6 inhibitors (fluoxetine, paroxetine, quinidine): May increase promethazine levels
- Levodopa: Promethazine may reduce effectiveness due to dopamine antagonism
- Anticholinesterases: Effects may be antagonized by promethazine
Clinical pearls
- Route matters: Promethazine should ideally be given orally or rectally. IM injection is preferred over IV due to severe tissue injury risk. If IV administration is necessary, use a large-bore vein, administer slowly, and monitor closely for extravasation.
- Pediatric risk: Beyond the black box warning for children under 2, use cautiously in all pediatric patients. Respiratory depression can occur at therapeutic doses, especially when combined with other CNS depressants.
- Anticholinergic toxicity: Classic presentation includes "hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter." Physostigmine is the antidote for severe cases.
- Extrapyramidal symptoms: Although less common than with typical antipsychotics, promethazine can cause acute dystonia, especially in children and young adults. Treat with diphenhydramine or benztropine.
- Not for chronic use: Promethazine is suitable for short-term management only. Chronic use leads to tolerance and increases risk of adverse effects.
- Pregnancy nausea: While historically used for hyperemesis gravidarum, safer alternatives (vitamin B6, doxylamine, ondansetron) are now preferred first-line.
- Masking signs: Can mask ototoxicity symptoms from other drugs and signs of intestinal obstruction or brain tumors due to antiemetic effects.
- Test interference: May interfere with pregnancy tests and cause false-positive urine glucose tests.
- Beers Criteria: Listed as potentially inappropriate medication in elderly due to high anticholinergic burden and fall risk.
Practice questions
1. A 14-month-old boy is brought to the emergency department with persistent vomiting for 24 hours. His mother states he has been unable to keep down fluids and appears lethargic. On examination, he has dry mucous membranes and decreased skin turgor. The physician considers prescribing promethazine for the vomiting. Which of the following best describes the use of promethazine in this patient?
2. A 68-year-old woman with Parkinson disease presents to the emergency department with nausea and vomiting after starting a new medication for motion sickness during a recent cruise. She reports feeling dizzy, having a dry mouth, and experiencing worsening tremor and rigidity. Her medication list includes carbidopa-levodopa, which she has been taking for 3 years. Physical examination reveals resting tremor, cogwheel rigidity, and bradykinesia. Which medication most likely caused her symptom exacerbation?
3. A 45-year-old man is brought to the emergency department after being found unresponsive. Paramedics report administering IV promethazine for nausea at an urgent care clinic 30 minutes prior. On arrival, his blood pressure is 70/40 mmHg and heart rate is 120/min. Physical examination reveals cool, pale extremities and a necrotic area on his right forearm where the IV was placed. Epinephrine IV is administered but his blood pressure drops further to 60/30 mmHg. Which of the following best explains this paradoxical response?
Frequently asked questions
How does promethazine work?
Promethazine works by blocking histamine H₁ receptors, which prevents allergic symptoms like itching and hives, and by blocking dopamine D₂ receptors and muscarinic receptors in the brain, which prevents nausea and causes sedation. It also has alpha-adrenergic blocking effects that can lower blood pressure. These multiple mechanisms make it useful for allergies, motion sickness, nausea, and short-term sedation, but also cause multiple side effects including drowsiness and dry mouth.
What are the side effects of promethazine?
Common side effects include drowsiness, dry mouth, blurred vision, constipation, and dizziness. Serious adverse effects include severe respiratory depression (especially in children), tissue damage and gangrene if IV infiltrates, dangerously low blood pressure, extrapyramidal symptoms like muscle spasms, seizures in susceptible individuals, and QT prolongation. Anticholinergic toxicity can occur with overdose. Elderly patients are at higher risk for confusion, falls, and urinary retention.
Is promethazine a controlled substance?
Promethazine itself is not a controlled substance in the United States. However, combination products containing promethazine and codeine are Schedule V controlled substances due to the codeine component. These combination syrups have been subject to misuse and diversion, leading to restricted prescribing in many jurisdictions. Plain promethazine requires a prescription but is not scheduled under the Controlled Substances Act.
Can promethazine be given IV?
Promethazine can be given intravenously but carries a black box warning for severe tissue injury including chemical irritation, thrombophlebitis, gangrene, and amputation if extravasation occurs or if inadvertently given intra-arterially. If IV administration is necessary, it must be through a secure, functioning large-bore vein and administered slowly. The preferred routes are oral, rectal, or intramuscular. Many institutions have restricted or banned IV promethazine due to these risks.
Promethazine vs ondansetron—what is the difference?
Promethazine is a first-generation antihistamine (phenothiazine) that blocks H₁, muscarinic, and dopamine receptors, causing significant sedation and anticholinergic side effects; it is used for nausea, allergies, and sedation. Ondansetron is a selective 5-HT₃ receptor antagonist with minimal sedation or anticholinergic effects, preferred for chemotherapy-induced and postoperative nausea. Ondansetron is safer in children and has no black box warnings for respiratory depression. Promethazine is contraindicated under age 2, while ondansetron can be used in infants. Ondansetron is generally preferred for antiemetic use due to its superior safety profile.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D04AA10WHO ATC/DDD index: R06AD02
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.