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 AND DEXTROMETHORPHAN HYDROBROMIDE
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 secretions, reducing mucus viscosity. Dextromethorphan is a centrally acting cough suppressant that binds to NMDA receptors and sigma-1 receptors, elevating the cough threshold.
Symptomatic relief of cough associated with upper respiratory tract infections,Allergic rhinitis,Motion sickness,Nausea and vomiting,Sedation
Temporary relief of cough due to minor throat and bronchial irritation (FDA-approved),Off-label: symptomatic treatment of upper respiratory tract infections with cough and congestion
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.
For adults and children ≥12 years: 10 m L (200 mg guaifenesin, 20 mg dextromethorphan) orally every 4 hours, not to exceed 60 m L (1200 mg guaifenesin, 120 mg dextromethorphan) per 24 hours.
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.
Guaifenesin: 1-2 hours; Dextromethorphan: 3-6 hours (extensive metabolizers), 18-24 hours (poor metabolizers due to CYP2D6 polymorphism).
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.
Guaifenesin is metabolized by oxidation and demethylation; dextromethorphan is extensively metabolized by CYP2D6 to dextrorphan (active metabolite) and other metabolites.
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%).
Guaifenesin: ~60% renal (metabolites), ~35% fecal; Dextromethorphan: ~70% renal (parent and metabolites, 45% as unchanged dextrorphan), ~20% biliary/fecal.
Promethazine: 93% bound primarily to albumin. Dextromethorphan: 60-70% bound to albumin and alpha-1-acid glycoprotein.
Guaifenesin: negligible (<10%); Dextromethorphan: ~60-70% (mainly albumin and alpha-1-acid glycoprotein).
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.
Guaifenesin: 1.2 L/kg (distributes into tissues); Dextromethorphan: 5-7 L/kg (large Vd due to high tissue binding).
Promethazine: oral 25% (extensive first-pass metabolism), intramuscular 100%, rectal 70-80%. Dextromethorphan: oral 11-60% (dependent on CYP2D6 metabolism), intramuscular not available.
Oral: Guaifenesin ~95%; Dextromethorphan ~11% (extensive first-pass metabolism, variable due to CYP2D6).
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 with caution in severe renal impairment (Cr Cl <30 m L/min) due to potential accumulation of dextromethorphan metabolite.
Child-Pugh A (mild): no adjustment. Child-Pugh B (moderate): reduce dose by 50% or prolong dosing interval. Child-Pugh C (severe): avoid use.
For dextromethorphan: Child-Pugh class C: consider reducing dose by 50% or avoid use; Child-Pugh A/B: no specific adjustment but monitor for CNS effects.
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 6-11 years: 5 m L (100 mg guaifenesin, 10 mg dextromethorphan) every 4 hours, max 30 m L/day. Children 2-5 years: 2.5 m L (50 mg guaifenesin, 5 mg dextromethorphan) every 4 hours, max 15 m L/day. Not for children <2 years.
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.
Use the lowest effective dose; consider starting with 5 m L (100 mg guaifenesin, 10 mg dextromethorphan) every 4-6 hours due to increased risk of sedation and anticholinergic 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
Avoid use in patients with chronic cough (e.g., smoking, asthma, emphysema) or cough with excessive phlegm.,Concomitant use with MAOIs or within 2 weeks of MAOI use is contraindicated.,Dextromethorphan abuse potential; use caution with CYP2D6 inhibitors.
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 dextromethorphan,Concurrent use or recent use (within 2 weeks) of monoamine oxidase inhibitors (MAOIs),Severe hypertension, coronary artery disease, or narrow-angle glaucoma (due to sympathomimetic effects if combined with decongestants; note: this combination alone does not contain decongestants, but caution applies)
Avoid grapefruit juice as it may increase dextromethorphan levels. No significant food interactions with promethazine.
No significant food interactions; avoid alcohol as it may increase sedation and dizziness.
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).
Guaifenesin: Limited human data; animal studies show no teratogenicity at clinically relevant doses. Dextromethorphan: No increased risk of major malformations in first trimester; animal studies show no teratogenicity. Avoid excessive doses in third trimester due to potential neonatal withdrawal or respiratory depression. Overall, both agents are considered low risk but use only if clearly needed.
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.
Guaifenesin: Excreted in breast milk in small amounts; unlikely to cause adverse effects in infants. Dextromethorphan: Excreted in breast milk; limited data suggest low infant exposure (M/P ratio not established). Both are considered compatible with breastfeeding; use lowest effective dose and monitor infant for sedation or respiratory depression.
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 pharmacokinetic data to support dose adjustments during pregnancy; use lowest effective dose for shortest duration. Guaifenesin: increased renal clearance in pregnancy may theoretically reduce efficacy, but no dose adjustment recommended. Dextromethorphan: metabolism by CYP2D6 may be affected by pregnancy; avoid exceeding standard doses.
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.
Monitor for sedation and dizziness, especially in elderly; avoid use with MAOIs due to serotonin syndrome risk; dextromethorphan has abuse potential at high doses; use caution in patients with chronic cough due to smoking, asthma, or COPD; guaifenesin may cause renal calculi with prolonged high doses.
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.
Do not exceed recommended doses; high doses can cause serious side effects including hallucinations and addiction.,Avoid driving or operating machinery if you feel dizzy or drowsy.,Do not use with other cough and cold medications to avoid overdose.,Increase fluid intake to help loosen mucus.,Stop use and consult a doctor if cough persists more than 7 days or comes with fever, rash, or headache.,Inform your doctor about all medications you take, especially MAOIs or SSRIs.,Keep out of reach of children; accidental overdose may be fatal in children.
"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."
"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."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about PROMETH W/ DEXTROMETHORPHAN vs GUAIFENESIN AND DEXTROMETHORPHAN HYDROBROMIDE, 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 AND DEXTROMETHORPHAN HYDROBROMIDE is a Expectorant/Antitussive Combination that works by Guaifenesin is an expectorant that increases respiratory tract fluid secretions, reducing mucus viscosity. Dextromethorphan is a centrally acting cough suppressant that binds to NMDA receptors and sigma-1 receptors, elevating the cough threshold.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between PROMETH W/ DEXTROMETHORPHAN and GUAIFENESIN AND DEXTROMETHORPHAN HYDROBROMIDE 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 AND DEXTROMETHORPHAN HYDROBROMIDE is: For adults and children ≥12 years: 10 m L (200 mg guaifenesin, 20 mg dextromethorphan) orally every 4 hours, not to exceed 60 m L (1200 mg guaifenesin, 120 mg dextromethorphan) per 24 hours.. Dosing should always be individualized based on indication, renal and hepatic function, age, and other patient factors.
A moderate-severity drug interaction has been identified when combining PROMETH W/ DEXTROMETHORPHAN and GUAIFENESIN AND DEXTROMETHORPHAN HYDROBROMIDE. 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. Consult your prescriber before combining these medications.
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 AND DEXTROMETHORPHAN HYDROBROMIDE is classified as Category C. Guaifenesin: Limited human data; animal studies show no teratogenicity at clinically relevant doses. Dextromethorphan: No increased risk of major malformations in first trimester; . Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.