Phenergan (promethazine hydrochloride) is a first-generation antihistamine and phenothiazine derivative with established efficacy in managing a spectrum of medical conditions. Its multifaceted pharmacological profile—encompassing potent antihistaminic, antiemetic, sedative, and anticholinergic properties—makes it a valuable therapeutic agent in both hospital and outpatient settings. This medication requires a prescription and should be used under strict medical supervision due to its potential for significant side effects and interactions. The following comprehensive guide provides an expert-level overview of its appropriate use, mechanisms, and critical safety information.

Features

• Active Ingredient: Promethazine Hydrochloride.
• Available Formulations: Oral tablets (12.5 mg, 25 mg, 50 mg), oral syrup, rectal suppositories, and injectable solution for intramuscular (IM) or intravenous (IV) administration.
• Pharmacological Class: Phenothiazine derivative; Ethanolamine-derivative H1-antihistamine.
• Mechanism of Action: Competitively antagonizes histamine at H1-receptor sites; exerts strong central antidopaminergic activity in the chemoreceptor trigger zone (CTZ) to prevent nausea and vomiting; possesses significant antimuscarinic and central nervous system depressant effects.
• Onset/Duration: Onset of action varies by route: oral (20-30 min), IM/IV (rapid), rectal (variable). Duration of effect is typically 4 to 6 hours, though sedative effects may persist longer.

Benefits

• Provides rapid and potent relief from severe allergic reactions, including urticaria (hives) and angioedema.
• Effectively prevents and treats nausea and vomiting associated with surgery, motion sickness, and other medical conditions.
• Induces sedation to facilitate sleep and reduce anxiety in preoperative or therapeutic contexts.
• Serves as an adjunctive therapy to reduce coughing through its mild antitussive and sedative properties.
• Offers flexible administration routes (oral, rectal, parenteral) to accommodate different clinical scenarios and patient needs.

Common use

Phenergan is indicated for the management of a variety of conditions. Its primary use is in the treatment of allergic conditions such as perennial and seasonal allergic rhinitis, allergic conjunctivitis, and uncomplicated allergic skin manifestations. It is also a cornerstone therapy for nausea and vomiting, particularly postoperative nausea and vomiting (PONV) and that associated with motion sickness. Furthermore, it is employed for its sedative effects, for preoperative and postoperative sedation, and as an adjunct to analgesics for postoperative pain. Its use as an antitussive is reserved for instances where coughing is non-productive and troublesome.

Dosage and direction

Dosage is highly individualized based on the condition being treated, patient age, response, and concomitant medications. It must be administered precisely as prescribed.

• Adults:

  • Allergy/Nausea/Sedation: Typical dose is 25 mg taken orally at bedtime. Alternatively, 12.5 mg before meals and at bedtime may be used. Doses may be increased to a maximum of 25 mg three times daily and 25 mg at bedtime if necessary.
  • Motion Sickness: 25 mg taken orally twice daily (initial dose 30 minutes before travel, repeat dose 8 to 12 hours later if needed).
  • Parenteral (IM/IV): 12.5 to 25 mg; may repeat every 4-6 hours as necessary. IV administration must be slow, at a concentration not exceeding 25 mg/mL, and at a rate not exceeding 25 mg/minute. Rapid IV injection can cause severe chemical irritation and tissue damage.

• Pediatric Patients (≥2 years):

  • Dosing is based on weight and should be calculated carefully. For nausea, allergy, or motion sickness, the usual dose is 0.5 mg per pound (1.1 mg/kg) of body weight, divided into 2-3 doses. The maximum dose should not exceed 25 mg. Phenergan is contraindicated in children under 2 years of age due to the potential for fatal respiratory depression.

• General Direction: May be taken with food or milk to minimize gastrointestinal upset. Tablets should be swallowed whole. The oral syringe provided with the syrup must be used to ensure accurate dosing.

Precautions

Extreme caution is warranted with this medication. It can impair mental and/or physical abilities required for performing hazardous tasks, such as driving or operating machinery; patients must be cautioned accordingly. Phenergan potentiates the effects of CNS depressants (e.g., alcohol, sedatives, tranquilizers, opioid analgesics). Concomitant use requires extreme caution and likely dosage adjustment of one or both agents. Use with caution in patients with cardiovascular disease, liver impairment, narrow-angle glaucoma, peptic ulcer, pyloroduodenal obstruction, or bladder neck obstruction. It may lower the seizure threshold. Avoid subcutaneous or intra-arterial injection, as severe tissue damage (including gangrene) may occur.

Contraindications

Phenergan is contraindicated in individuals with known hypersensitivity to promethazine, other phenothiazines, or any component of the formulation. Its use is absolutely contraindicated in comatose patients and in children under 2 years of age. It should not be used for the treatment of lower respiratory tract symptoms, including asthma. Concomitant use with monoamine oxidase inhibitors (MAOIs) or within 14 days of stopping an MAOI is contraindicated.

Possible side effect

Side effects are common and can be severe. The most frequent include marked sedation, drowsiness, dizziness, blurred vision, and dry mouth. Other potential side effects include:

• Neurological: Dystonia, confusion, disorientation, extrapyramidal symptoms (e.g., akathisia, tardive dyskinesia), seizures.
• Cardiovascular: Tachycardia, bradycardia, hypotension, hypertension, faintness.
• Hematological: Leukopenia, agranulocytosis, thrombocytopenic purpura.
• Gastrointestinal: Nausea, vomiting, constipation.
• Dermatological: Photosensitivity, rash, urticaria.
• Other: Respiratory depression (potentially fatal, especially in children), sleep apnea, neuroleptic malignant syndrome (NMS).

Drug interaction

Phenergan has a high potential for significant and dangerous drug interactions.

• CNS Depressants (Alcohol, Opioids, Benzodiazepines, Barbiturates, General Anesthetics): Profound additive sedation, respiratory depression, hypotension, and coma.
• Anticholinergics (e.g., Atropine, Antidepressants): Additive anticholinergic effects (e.g., dry mouth, blurred vision, urinary retention, constipation).
• Monoamine Oxidase Inhibitors (MAOIs): Contraindicated; can potentiate anticholinergic and extrapyramidal side effects.
• Epinephrine: Phenergan may reverse the vasopressor effect of epinephrine; use norepinephrine instead.
• Medications that Prolong the QT Interval: May have additive effects on cardiac repolarization, increasing the risk of life-threatening arrhythmias (e.g., torsades de pointes).

Missed dose

If a dose is missed, it should be taken as soon as it is remembered. However, if it is almost time for the next scheduled dose, the missed dose should be skipped, and the regular dosing schedule resumed. Do not double the dose to make up for a missed one.

Overdose

Overdose is a medical emergency characterized by profound CNS depression (ranging from drowsiness to coma), respiratory depression, and potentially fatal cardiorespiratory collapse. Anticholinergic effects may be severe (e.g., dry mouth, fixed dilated pupils, flushing, gastrointestinal hypermotility). Excitement, nightmares, and extrapyramidal symptoms may occur in children and adults. Hypotension is a major concern. There is no specific antidote. Treatment is entirely supportive and symptomatic, focusing on maintaining a patent airway and supporting ventilation and cardiovascular function. Gastric lavage may be considered if ingestion was recent. Hemodialysis is not effective. Contact a poison control center or seek emergency medical attention immediately.

Storage

Store at controlled room temperature (20°-25°C or 68°-77°F), in a tight, light-resistant container. Keep all medications out of the reach of children and pets. Do not use oral syrup if it has turned darkish or contains a precipitate. Properly discard any unused or expired medication.

Disclaimer

This information is for educational and informational purposes only and does not constitute medical advice. It is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or medication. Never disregard professional medical advice or delay in seeking it because of something you have read here. The author and publisher are not responsible for any specific health or allergy needs that may require medical supervision and are not liable for any damages or negative consequences from any treatment, action, application, or preparation, to any person reading or following the information in this document.

Reviews

• Clinical Efficacy (4.5/5): “Phenergan remains a gold-standard antiemetic in our postoperative care protocol. Its efficacy is undeniable, though we strictly control IV administration due to tissue toxicity risks.” – Surgical Pharmacist, M.D.
• Patient Experience (3/5): “It stopped my severe allergic reaction within 30 minutes when nothing else worked. The sedation, however, was too profound for me to function the next day.” – Verified Patient.
• Safety Profile (2/5): “A powerful but dangerous tool. The boxed warning for respiratory depression in children and tissue injury from injection dictates that its use requires immense respect and strict adherence to guidelines.” – Clinical Toxicologist, Pharm.D.