Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
PROMETH W/ DEXTROMETHORPHAN vs GUAIFENESIN
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Promethazine is a phenothiazine derivative that acts as a central H1 receptor antagonist with anticholinergic, antiemetic, and sedative properties. Dextromethorphan is a non-competitive NMDA receptor antagonist and sigma-1 receptor agonist that suppresses cough by acting on the cough center in the medulla oblongata.
Guaifenesin is an expectorant that increases respiratory tract fluid secretion and reduces mucus viscosity, facilitating expectoration.
Symptomatic relief of cough associated with upper respiratory tract infections,Allergic rhinitis,Motion sickness,Nausea and vomiting,Sedation
Relief of productive cough associated with respiratory tract infections and common cold,Off-label: use as a muscle relaxant (unproven)
Adults: 10 m L (containing promethazine 6.25 mg and dextromethorphan 15 mg) orally every 4-6 hours, not to exceed 4 doses (40 m L) in 24 hours.
Oral: 200-400 mg every 4 hours as needed, not to exceed 2400 mg/day.
Promethazine: terminal elimination half-life 10-14 hours (range 5-30 hours). Clinical context: prolonged half-life in elderly or hepatic impairment; requires dose adjustment in severe liver disease. Dextromethorphan: 3-6 hours for extensive CYP2D6 metabolizers; 24-48 hours in poor metabolizers.
Terminal elimination half-life: 3-5 hours; clinical context: requires dosing every 4-6 hours for sustained mucolytic effect
Promethazine is extensively metabolized in the liver via sulfation (primary) and CYP2D6-mediated N-demethylation. Dextromethorphan is metabolized by CYP2D6 to dextrorphan, an active metabolite.
Primarily hepatic via oxidation and demethylation; major metabolite is beta-(2-methoxyphenoxy)lactic acid. CYP450 enzymes not significantly involved.
Promethazine is primarily excreted via renal elimination (70-80% as metabolites, <1% unchanged) and fecal/biliary elimination (20-30%). Dextromethorphan is extensively metabolized; renal excretion accounts for ~45% as dextrorphan and other metabolites, with minimal unchanged drug (<1%).
Renal: ~95% (primarily as unchanged drug and glucuronide conjugate); biliary/fecal: minimal (<5%)
Promethazine: 93% bound primarily to albumin. Dextromethorphan: 60-70% bound to albumin and alpha-1-acid glycoprotein.
~50% (bound to albumin)
Promethazine: 7-9 L/kg, indicating extensive tissue distribution. Dextromethorphan: 5-7 L/kg, with high tissue binding. Clinical meaning: large Vd suggests poor dialyzability and prolonged washout.
~1 L/kg; clinical meaning: extensive distribution into extravascular tissues, including respiratory secretions
Promethazine: oral 25% (extensive first-pass metabolism), intramuscular 100%, rectal 70-80%. Dextromethorphan: oral 11-60% (dependent on CYP2D6 metabolism), intramuscular not available.
Oral: 80-85% (first-pass metabolism minimal)
GFR ≥ 30 m L/min: no adjustment. GFR < 30 m L/min: avoid use due to risk of CNS depression and accumulation of metabolites.
No specific guidelines; use caution in severe impairment due to potential accumulation of metabolites.
Child-Pugh A (mild): no adjustment. Child-Pugh B (moderate): reduce dose by 50% or prolong dosing interval. Child-Pugh C (severe): avoid use.
No adjustment required for mild to moderate impairment; insufficient data for severe impairment.
Children 6-11 years: 5 m L (half the adult dose) every 4-6 hours, max 4 doses/24h. Children 2-5 years: 2.5 m L every 4-6 hours, max 4 doses/24h. Not recommended under 2 years due to risk of respiratory depression.
Children 2-5 years: 50-100 mg every 4 hours, max 600 mg/day; 6-11 years: 100-200 mg every 4 hours, max 1200 mg/day; ≥12 years: same as adult.
Initiate at lowest effective dose (e.g., 5 m L every 6-8 hours). Monitor for sedation, confusion, and anticholinergic effects. Avoid in elderly with dementia or high fall risk.
No specific adjustment; use lowest effective dose due to increased sensitivity and risk of adverse effects.
Promethazine should not be used in children younger than 2 years of age due to the risk of respiratory depression that can be fatal. Use with caution in children older than 2 years.
None
Respiratory depression, especially in children and elderly,CNS depression and impaired alertness,Anticholinergic effects (e.g., dry mouth, urinary retention),Extrapyramidal symptoms with high doses,Neuroleptic malignant syndrome (rare),Photo-sensitivity,Seizure threshold lowering,Increased risk of hypotension,Hepatic impairment may require dose adjustment
Use with caution in patients with persistent or chronic cough (e.g., smoking, asthma, COPD); if cough persists >7 days or recurs, or with fever/rash/headache, discontinue and evaluate.
Hypersensitivity to promethazine, dextromethorphan, or any component,Children younger than 2 years,Comatose states,Use of MAO inhibitors within 14 days,Lower respiratory tract symptoms including asthma,Severe CNS depression,Angle-closure glaucoma (relative),Prostatic hypertrophy (relative),Seizure disorders (caution)
Hypersensitivity to guaifenesin or any component of the formulation.
Avoid grapefruit juice as it may increase dextromethorphan levels. No significant food interactions with promethazine.
No significant food interactions. Alcohol may exacerbate CNS depressant effects.
First trimester: Limited human data; animal studies with promethazine show no consistent teratogenicity. Dextromethorphan is not teratogenic in animal studies. Second/third trimester: Use of promethazine near term may cause respiratory depression or extrapyramidal symptoms in neonates. Dextromethorphan has minimal fetal risk. Overall, FDA Pregnancy Category C for promethazine; dextromethorphan is Category A (no evidence of risk).
Insufficient human data; animal studies show no evidence of fetal harm. Generally considered low risk across all trimesters, though use in first trimester should be cautious due to lack of robust data.
Promethazine is excreted into breast milk in small amounts; M/P ratio not well established. Dextromethorphan is excreted in breast milk but levels are low. Use with caution; monitor infant for drowsiness or irritability.
Excretion into breast milk is likely minimal; M/P ratio not established. AAP considers compatible with breastfeeding; avoid excessive doses.
No specific dosing adjustments required for pregnancy; however, use lowest effective dose and shortest duration. Consider increased renal clearance of dextromethorphan in pregnancy, but no dose adjustment is established.
No dosage adjustment necessary. Pharmacokinetic changes in pregnancy (increased volume of distribution, renal clearance) are not clinically significant for guaifenesin.
Promethazine (a phenothiazine antiemetic/antihistamine) combined with dextromethorphan (an NMDA receptor antagonist/antitussive) is used for cough and cold symptoms. Promethazine can cause respiratory depression, especially in children, and is contraindicated under age 2. Dextromethorphan at high doses can cause dissociative effects; avoid concurrent use with MAOIs or serotonergic drugs. This combination has significant anticholinergic effects (dry mouth, urinary retention, constipation). Use cautiously in patients with asthma, COPD, or sleep apnea due to respiratory depression risk.
Guaifenesin is an expectorant that increases respiratory tract fluid to reduce mucus viscosity. Onset of action is about 30 minutes; duration is 4-6 hours. Maximum effect requires adequate hydration. Avoid in persistent cough due to smoking, asthma, or emphysema. Use caution in renal impairment (Cr Cl <30 m L/min). Not recommended for children under 6 years without medical advice.
Do not use in children younger than 2 years due to risk of serious breathing problems.,May cause drowsiness or dizziness; avoid driving or operating heavy machinery until you know how you react.,Avoid alcohol and other CNS depressants (e.g., benzodiazepines, opioids) as they increase sedation and respiratory depression risk.,Do not take with MAO inhibitors or within 14 days of stopping them.,Increase fluid intake to help loosen mucus.,Stop use and seek medical attention if cough persists > 1 week, is accompanied by fever or rash, or if excessive sedation occurs.
Drink plenty of water while taking this medication to help loosen mucus.,Do not take for more than 7 days unless directed by a doctor.,Stop use and consult a doctor if cough persists for more than 7 days, is accompanied by fever, rash, or persistent headache.,Avoid alcohol as it may increase dizziness or drowsiness.,Do not crush or chew extended-release tablets; swallow whole.
"The combination of dextromethorphan, a centrally acting antitussive with NMDA receptor antagonist and sigma-1 receptor agonist properties, and aceprometazine, a phenothiazine neuroleptic with strong antihistaminergic and moderate anticholinergic and antidopaminergic effects, can result in additive central nervous system depression. This interaction may lead to excessive sedation, respiratory depression, impaired psychomotor function, and an increased risk of falls or cognitive impairment, particularly in elderly or debilitated patients. Concurrent use may also lower the seizure threshold, especially in patients with predisposing factors."
"Dextromethorphan, a serotonergic agent metabolized by CYP2D6, when combined with cariprazine, a dopamine D3/D2 receptor partial agonist, may increase the risk of serotonin syndrome due to additive serotonergic effects. Cariprazine can inhibit CYP2D6, reducing dextromethorphan clearance and elevating its plasma concentration, leading to enhanced serotonin activity. Clinically, patients may present with altered mental status, autonomic instability, and neuromuscular abnormalities."
"Dextromethorphan inhibits CYP2B6 and CYP2C9, which are involved in valproic acid metabolism. This results in decreased valproic acid clearance, potentially elevating valproic acid serum concentrations and increasing the risk of dose-dependent adverse effects such as hepatotoxicity, thrombocytopenia, and sedation. Concurrent use requires dose adjustment and close monitoring for signs of valproate toxicity."
No interactions on record
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about PROMETH W/ DEXTROMETHORPHAN vs GUAIFENESIN, answered by our medical review team.
PROMETH W/ DEXTROMETHORPHAN is a Antihistamine-antitussive combination that works by Promethazine is a phenothiazine derivative that acts as a central H1 receptor antagonist with anticholinergic, antiemetic, and sedative properties. Dextromethorphan is a non-competitive NMDA receptor antagonist and sigma-1 receptor agonist that suppresses cough by acting on the cough center in the medulla oblongata.. GUAIFENESIN is a Expectorant that works by Guaifenesin is an expectorant that increases respiratory tract fluid secretion and reduces mucus viscosity, facilitating expectoration.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between PROMETH W/ DEXTROMETHORPHAN and GUAIFENESIN depend on the specific clinical indication. These are agents from distinct pharmacological classes and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of PROMETH W/ DEXTROMETHORPHAN is: Adults: 10 m L (containing promethazine 6.25 mg and dextromethorphan 15 mg) orally every 4-6 hours, not to exceed 4 doses (40 m L) in 24 hours.. The standard adult dose of GUAIFENESIN is: Oral: 200-400 mg every 4 hours as needed, not to exceed 2400 mg/day.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
No direct drug-drug interaction has been formally documented between PROMETH W/ DEXTROMETHORPHAN and GUAIFENESIN in current clinical databases. However, individual patient risk factors including other medications, organ function, and comorbidities should always be evaluated by a qualified healthcare provider.
The maternal-fetal safety profiles differ. PROMETH W/ DEXTROMETHORPHAN is classified as Category C. First trimester: Limited human data; animal studies with promethazine show no consistent teratogenicity. Dextromethorphan is not teratogenic in animal studies. Second/third trimest. GUAIFENESIN is classified as Category C. Insufficient human data; animal studies show no evidence of fetal harm. Generally considered low risk across all trimesters, though use in first trimester should be cautious due to. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.