Phenergan
Phenergan
Generic Name
Phenergan
Mechanism
- H1 Antagonism – competitively blocks histamine H1 receptors on the central and peripheral nervous system, reducing pruritus, urticaria, and edema.
- Anticholinergic Activity – inhibits muscarinic acetylcholine receptors, contributing to dry mouth, blurred vision, and urinary retention.
- Anti‑emetic Effect – acts on the vestibular apparatus and the chemoreceptor trigger zone (CTZ) in the medulla, attenuating nausea and vomiting.
- Dopaminergic Antagonism – blocks dopamine D2 receptors centrally, which contributes to its anti‑emetic and anti‑nausea effectiveness, and may modestly affect gastric motility.
- CNS Depressant Properties – crosses the blood–brain barrier and centrally inhibits neuronal firing, providing sedation and mild analgesic‑adjuvant effects.
Pharmacokinetics
| Parameter | Details |
| Absorption | Oral bioavailability ~70–80 % ; peak plasma levels in 0.5–1 h. |
| Distribution | Highly lipophilic; extensive tissue distribution; protein binding 60–80 %. |
| Metabolism | Hepatic via CYP2D6 and CYP3A4 to inactive metabolites. |
| Elimination | Renal excretion 80–90 % of dose; elimination half‑life 4–6 h (oral), 5–8 h (IV). |
| Special Populations | Slower clearance in hepatic impairment, potentially prolonged effects. |
| Pregnancy & Lactation | Category C; crosses placenta and is excreted in breast milk; caution advised. |
Indications
- Allergic Disorders – urticaria, allergic rhinitis, rhinosinusitis, anaphylaxis adjunct.
- Anti‑emetic Therapy – motion sickness, postoperative nausea/vomiting, chemotherapy‑induced emesis.
- Sedation & Antiemetic Adjunct – pre‑operative sedation for minor surgeries, especially in pediatric patients; adjunct with opioids for severe pain.
- Dermatology – topical analgesic, anti‑pruritic in certain dermatoses (e.g., skin rashes).
- Symptomatic Relief – colic in infants (under careful supervision due to CNS suppression).
Contraindications
- Absolute Contraindications – known hypersensitivity to promethazine or any excipients; severe CNS depression, narrow‑angle glaucoma, myasthenia gravis; severe hepatic impairment; pregnancy (trimester 1).
- Relative Contraindications – seizure disorders (due to lowered seizure threshold), severe respiratory insufficiency, recent NSAID use (risk of bleeding), elderly patients (due to anticholinergic burden).
- Warnings – risk of respiratory depression, especially with opioids or alcohol; QT‑interval prolongation (monitor ECG in high‑dose or persistently therapy); hepatotoxicity in prolonged use; potential for excessive sedation and falls in older adults.
Dosing
| Route | Adult Dose (Typical) | Pediatric Dose | Max Daily Dose |
| Oral | 25–50 mg q 6–8 h (max 400 mg/day) | 0.1–0.3 mg/kg q6–8 h (max 1 mg/kg/day) | 400 mg |
| IV | 12.5–25 mg over 30 min (repeatable); max 100 mg/day | 0.05–0.1 mg/kg q 4–6 h (max 0.3 mg/kg/day) | 100 mg |
| IM | 25 mg singly (may repeat 1–2 ×) | 0.1 mg/kg once | 100 mg |
| Topical | 2–8 mg (½–⅓ tsp.) on affected area | 0.3–1 mg/kg of skin area | 8 mg |
• Administration Tips:
• IV infusion shall be no faster than 25 mg per 30 min to avoid hypotension.
• Oral dosing may be delayed after meals to improve absorption.
• In patients with hepatic dysfunction, consider a reduced dose and monitor hepatic enzymes.
Monitoring
- Vital Signs – blood pressure, heart rate, respiratory rate, SpO₂, especially during IV therapy.
- Liver Function Tests (LFTs) – baseline and periodic if therapy >1 week.
- Electrocardiogram (ECG) – baseline & follow‑up in patients on high doses or with pre‑existing cardiac disease.
- Sedation Assessment – RASS or sedation scales for ICU or peri‑operative settings.
- Patient‑Reported Symptoms – any signs of fetal or neonatal depression if breastfeeding.
Clinical Pearls
- Pediatric Caution: Use of promethazine in the pediatric age group demands careful monitoring for respiratory depression; avoid in children <1 yr unless necessary and under strict supervision.
- Alcohol Interaction: Co‑administration with alcoholic beverages markedly increases CNS depression; counsel patients to avoid alcohol during and immediately after therapy.
- Drug‑Drug Alerts: Combining with other CNS depressants (benzodiazepines, opioids, sedatives) or anticholinergics (antimuscarinics) can precipitate severe sedation, hypotension, or anticholinergic toxicity.
- Pregnancy & Lactation: Category C; avoid use in the first trimester and cautious use during lactation—promethazine passes into breast milk and can cause sedation in toddlers.
- QT‑Prolongation Screening: Patients with congenital long QT syndrome, use of other QT‑prolonging agents (macrolides, fluoroquinolones), or electrolyte imbalances should be screened prior to prescribing.
- Use in Pre‑operative Sedation: While effective, it is less favored now for major surgeries due to additive respiratory suppression with anesthetic agents; consider alternatives such as dexmedetomidine or midazolam when possible.
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• Key Takeaway: Phenergan is an indispensable antihistamine‑antiemetic, yet its broad pharmacologic spectrum demands vigilant dosing, monitoring, and patient education, especially in vulnerable populations such as infants, the elderly, and those on concurrent CNS depressants.