Phenergan: Expert Relief for Severe Allergies and Nausea
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Synonyms
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Phenergan (promethazine hydrochloride) is a first-generation antihistamine and phenothiazine derivative with established efficacy in managing severe allergic reactions, motion sickness, and postoperative nausea. As a prescription medication with potent sedative and antiemetic properties, it operates through central histamine H1-receptor antagonism and muscarinic acetylcholine receptor blockade. Its multifaceted pharmacological profile makes it a versatile agent in clinical practice, though its use requires careful patient selection and monitoring due to potential CNS depression. This product card provides a comprehensive, evidence-based overview for healthcare professionals and informed patients.
Features
- Active ingredient: Promethazine hydrochloride
- Available formulations: Oral tablets (12.5 mg, 25 mg, 50 mg), syrup (6.25 mg/5 mL), rectal suppositories (12.5 mg, 25 mg), injectable solution (25 mg/mL, 50 mg/mL)
- Mechanism: Potent H1-histamine receptor antagonist with additional anticholinergic, antiemetic, and sedative effects
- Onset of action: Oral: 20 minutes; IV: 3-5 minutes; Duration: 4-6 hours (up to 12 hours for sedative effects)
- Pregnancy category: C (risk cannot be ruled out)
- Half-life: 9-16 hours (prolonged in hepatic impairment)
- Metabolism: Hepatic via CYP2D6 and CYP2B6; significant first-pass effect
- Excretion: Primarily renal (as inactive metabolites)
Benefits
- Provides rapid relief from severe allergic reactions including urticaria, angioedema, and anaphylaxis (as adjunct therapy)
- Effectively prevents and treats nausea and vomiting associated with motion sickness, surgery, or chemotherapy
- Offers pronounced sedative effects for preoperative anxiety and postoperative rest
- Demonstrates potent antitussive properties for debilitating cough
- Serves as an adjunct to analgesics for pain management through sedation enhancement
- Provides cost-effective symptomatic relief across multiple clinical indications
Common use
Phenergan is clinically indicated for the management of allergic conditions including perennial and seasonal allergic rhinitis, vasomotor rhinitis, and allergic conjunctivitis. It is extensively used for motion sickness prevention and treatment, particularly in aviation and maritime contexts. The medication serves as an effective antiemetic for postoperative nausea and vomiting, radiation sickness, and as adjunct therapy in opioid-induced nausea. Its sedative properties make it valuable for preoperative sedation, postoperative rest, and as a nighttime sleep aid. Off-label uses include management of migraine-associated nausea, vertigo, and as an adjunct in anaphylaxis treatment.
Dosage and direction
Adults: Allergic conditions: 12.5 mg before bed and 6.25-12.5 mg twice daily; may increase to 25 mg at bedtime. Motion sickness: 25 mg 30-60 minutes before travel, then 12.5-25 mg every 8-12 hours. Nausea/vomiting: 12.5-25 mg every 4-6 hours. Sedation: 25-50 mg at bedtime. Pediatric (≥2 years): 0.25-0.5 mg/kg/dose every 6 hours (max 25 mg). Elderly: Initiate at lower end of dosing range (6.25-12.5 mg) due to increased sensitivity. Administration: Oral tablets should be swallowed whole with water. Syrup should be measured precisely using provided device. Injectable form must be administered deep IM or by slow IV infusion (max 25 mg/mL concentration, rate not exceeding 25 mg/min). Avoid subcutaneous or intra-arterial administration.
Precautions
Phenergan may cause significant drowsiness; patients should avoid driving or operating machinery until response is known. Concurrent CNS depressants (alcohol, opioids, benzodiazepines) potentiate sedation. Use with caution in patients with respiratory depression, asthma, or COPD due to potential suppression of cough reflex. Monitor for orthostatic hypotension, particularly in elderly and volume-depleted patients. Caution in cardiovascular disease due to potential tachycardia and hypotension. Risk of paradoxical excitation in children and elderly. Photosensitivity reactions may occur. Tissue damage and gangrene possible with perivascular extravasation of IV formulation. Regular liver function monitoring recommended with prolonged use.
Contraindications
Absolute contraindications include known hypersensitivity to promethazine or phenothiazines, coma states, and concomitant MAOI therapy (risk of hypertensive crisis). Contraindicated in children under 2 years due to risk of fatal respiratory depression. Avoid in patients with narrow-angle glaucoma, bladder neck obstruction, pyloroduodenal obstruction, and stenosing peptic ulcer. Do not administer intra-arterially or subcutaneously. Contraindicated in patients with bone marrow suppression, severe liver impairment, or sleep apnea. Not recommended during breastfeeding due to secretion into breast milk.
Possible side effect
Common (≥1%): Sedation, drowsiness, blurred vision, dry mouth, dizziness, hypotension, tinnitus, incoordination. Less common (0.1-1%): Paradoxical excitation (especially in children), nightmares, restlessness, extrapyramidal symptoms, photosensitivity, weight gain, constipation, urinary retention. Rare (<0.1%): Blood dyscrasias (agranulocytosis, leukopenia), neuroleptic malignant syndrome, seizures, jaundice, arrhythmias, tissue necrosis with IV extravasation. Pediatric-specific: Respiratory depression, apnea, sudden infant death syndrome (in children under 2), excitability. Elderly-specific: Increased risk of falls, confusion, anticholinergic effects.
Drug interaction
Major interactions: CNS depressants (opioids, benzodiazepines, alcohol) → additive sedation and respiratory depression. MAOIs → hypertensive crisis. Anticholinergics → enhanced anticholinergic toxicity. Epinephrine → reversed pressor effect. Moderate interactions: CYP2D6 inhibitors (fluoxetine, paroxetine) → increased promethazine levels. Hypotensive agents → additive hypotension. QT-prolonging agents → increased arrhythmia risk. Laboratory interactions: May cause false-positive pregnancy tests, interfere with urinary steroid determinations, and increase blood glucose levels.
Missed dose
If a dose is missed, administer as soon as remembered unless it is nearly time for the next scheduled dose. Do not double the dose to make up for a missed administration. For once-daily dosing regimens, if remembered within 12 hours of scheduled time, take immediately; if beyond 12 hours, skip and resume normal schedule. For multiple daily dosing, take if remembered within 4 hours of scheduled time; otherwise, skip and resume normal schedule. Consistent timing is important for motion sickness prevention.
Overdose
Symptoms: Severe CNS depression (coma, respiratory depression), hypotension, tachycardia, anticholinergic crisis (hyperthermia, flushed skin, dry mouth), extrapyramidal symptoms, seizures, cardiorespiratory arrest. Management: Immediate medical attention required. Gastric lavage if recent ingestion. Activated charcoal may be beneficial. Supportive care including airway management, ventilatory support, and IV fluids for hypotension. Avoid analeptics. Treat seizures with benzodiazepines. Extrapyramidal symptoms may respond to diphenhydramine or benztropine. Hemodialysis not effective due to high protein binding. Monitor ECG for QT prolongation.
Storage
Store at controlled room temperature (20-25°C/68-77°F). Protect from light and moisture. Keep oral formulations in tightly closed containers. Injectable solution should be protected from light until administration. Do not freeze. Keep out of reach of children and pets. Discard unused portion of oral solution after 6 months of opening. Do not use if discolored or containing precipitate. Properly dispose of unused medication through take-back programs.
Disclaimer
This information is for educational purposes only and does not constitute medical advice. Phenergan is a prescription medication requiring professional supervision. Individual response may vary. Always consult a healthcare provider for proper diagnosis, dosing, and monitoring. Do not initiate or discontinue without medical guidance. Full prescribing information should be reviewed before administration. Report adverse events to FDA MedWatch.
Reviews
“Phenergan remains our go-to for severe allergic presentations in the ED. Its rapid onset and multifaceted action make it invaluable, though we’re meticulous about respiratory monitoring.” - Emergency Physician, 15 years experience
“As an oncologist, I appreciate Phenergan’s antiemetic efficacy for chemotherapy-induced nausea. The sedative effect is particularly beneficial for patients experiencing treatment-related anxiety.” - Oncology Specialist
“While effective, the sedation profile requires careful patient education. We’ve implemented strict fall-risk protocols for elderly patients prescribed Phenergan.” - Geriatric Pharmacist
“The IV formulation’s vesicant properties demand expert administration. Our institution requires central line administration and frequent site assessment.” - ICU Nurse Educator
“Despite newer agents, Phenergan’s cost-effectiveness and reliability maintain its place in our formulary for appropriate patients with severe motion sickness.” - Travel Medicine Specialist