Head-to-head clinical analysis & difference comparison: details on mechanism of action, dosing, half-life, interactions, and maternal-fetal safety.
NORPACE vs DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE
Clinician-reviewed, head-to-head comparison of mechanism, dosing, pharmacokinetics, and safety profiles.
Last clinically reviewed: July 2026 · OpiCalc Medical Review Team
Class Ic antiarrhythmic agent; blocks voltage-gated sodium channels, slowing conduction velocity and prolonging refractory periods in cardiac tissue.
Dextromethorphan is an uncompetitive NMDA receptor antagonist and sigma-1 receptor agonist; quinidine is a CYP2D6 inhibitor that increases dextromethorphan bioavailability.
Treatment of documented life-threatening ventricular arrhythmias, such as sustained ventricular tachycardia,Off-label: Atrial fibrillation, atrial flutter, supraventricular tachycardia
Pseudobulbar affect (PBA) - FDA approved
150 mg orally every 6 hours (maximum 300 mg per dose), extended-release formulation 300 mg every 12 hours.
One capsule (dextromethorphan 20 mg/quinidine 10 mg) orally once daily, with a maximum dose of two capsules per day.
Terminal elimination half-life: 6-8 hours (normal renal function); prolonged in renal impairment (up to 24 hours).
Dextromethorphan: 2-4 hours (extensive metabolizers); quinidine: 6-8 hours (inhibits CYP2D6, prolonging dextromethorphan half-life in poor metabolizers to >20 hours)
Extensively metabolized in the liver primarily by CYP2D6; also involves CYP1A2 and CYP3A4 to a minor extent. Active metabolite: desethylnorpace.
Dextromethorphan is primarily metabolized by CYP2D6 to dextrorphan; quinidine is a potent CYP2D6 inhibitor at low doses.
Renal: 40-60% unchanged; biliary/fecal: minor (10-20%).
Renal: quinidine 15-25% unchanged, dextromethorphan <1% unchanged; biliary/fecal: quinidine metabolites ~5%, dextromethorphan metabolites ~60-80% as dextrorphan conjugates
80-90%, primarily to alpha-1-acid glycoprotein and albumin.
Dextromethorphan: ~60-70% (albumin); quinidine: 80-90% (albumin, alpha-1-acid glycoprotein)
1.8-3.6 L/kg; large, indicating extensive tissue distribution.
Dextromethorphan: 5-6 L/kg; quinidine: 2-3 L/kg
Oral: 80-90%.
Oral: dextromethorphan ~11% (extensive first-pass metabolism; increased to >50% when coadministered with quinidine); quinidine ~70-80%
GFR 30-50 m L/min: 150 mg every 12-24 hours; GFR 15-29 m L/min: 150 mg every 24-48 hours; GFR <15 m L/min (not on dialysis): 150 mg every 48 hours or 100 mg every 24 hours.
For creatinine clearance (Cr Cl) 30–59 m L/min: reduce dose to one capsule once daily. For Cr Cl <30 m L/min: not recommended.
Child-Pugh Class A: no adjustment; Child-Pugh Class B: reduce dose by 25-50% with monitoring; Child-Pugh Class C: contraindicated or use with extreme caution.
Child-Pugh Class A: no adjustment. Child-Pugh Class B or C: not recommended (quinidine is extensively metabolized by the liver).
<1 year: 10-30 mg/kg/day divided every 6 hours; 1-4 years: 10-30 mg/kg/day divided every 6 hours; 4-12 years: 10-30 mg/kg/day divided every 6 hours; 12-18 years: 150-300 mg every 6 hours. Maximum 800 mg/day.
Safety and efficacy in pediatric patients have not been established; not recommended for use in children.
Initiate at 100 mg every 6 hours; consider lower doses due to age-related renal decline; monitor for anticholinergic effects and QT prolongation.
Initiate at one capsule once daily; titrate cautiously. Monitor for QT prolongation and anticholinergic effects. Lower doses may be required due to age-related renal and hepatic decline.
None
None.
Proarrhythmic effects (e.g., new or worsened arrhythmias, torsades de pointes),Heart failure exacerbation,Hepatic impairment,Renal impairment,Electrolyte disturbances (hypokalemia, hypomagnesemia),Conduction disturbances (e.g., QT prolongation, heart block)
CNS depression: may cause dizziness, somnolence; avoid alcohol.,Cardiotoxicity: quinidine may cause QT prolongation; monitor ECG.,Serotonin syndrome: risk with concurrent serotonergic drugs.,Hepatic impairment: use with caution.,Renal impairment: not recommended in severe renal disease.
Pre-existing second- or third-degree AV block unless pacemaker is present,Cardiogenic shock,Severe heart failure,QTc interval > 450 ms,Concomitant use of other QT-prolonging drugs,Hypersensitivity to disopyramide or any component
Concomitant use with monoamine oxidase inhibitors (MAOIs) or within 14 days.,Concomitant use with other drugs that prolong QT interval.,Known hypersensitivity to dextromethorphan or quinidine.,Complete atrioventricular block without pacemaker.,History of drug-induced torsades de pointes.
Grapefruit juice may increase disopyramide levels; avoid concurrent intake. High-fat meals may delay absorption; take consistently with or without food. Avoid excessive alcohol, which can exacerbate hypotension and arrhythmias.
Avoid grapefruit and grapefruit juice as they may increase plasma levels of quinidine, raising risk of toxicity including QT prolongation. Avoid excessive alcohol consumption as it may exacerbate CNS depression.
First trimester: No evidence of increased risk of congenital malformations in human studies, but animal studies are insufficient. Second and third trimesters: Risk of fetal bradycardia, QT prolongation, and neonatal depression at delivery. Disopyramide may stimulate uterine contractions, increasing risk of preterm labor.
First trimester: Limited human data; animal studies show quinidine sulfate associated with increased risk of fetal malformations at high doses. Second/third trimester: Dextromethorphan hydrobromide not associated with major malformations; quinidine sulfate may cause fetal bradycardia and hypoglycemia due to quinidine's antiarrhythmic effects. Overall: No adequate human studies; use only if benefit outweighs risk.
Disopyramide is excreted in breast milk with an M/P ratio of approximately 1:1.1. The relative infant dose is about 2–10% of the maternal weight-adjusted dose. Monitor infant for bradycardia, QT changes, and hypoglycemia. Alternative agents preferred unless benefit outweighs risk.
Dextromethorphan: Likely excreted in breast milk in low amounts; M/P ratio not reported. Quinidine: Excreted in breast milk; M/P ratio approximately 0.71. Potential for infant quinidine toxicity (cinchonism, arrhythmias). Avoid breastfeeding during maternal quinidine therapy.
Increased renal clearance and volume of distribution in pregnancy may reduce disopyramide serum concentrations. Therapeutic drug monitoring recommended; dose adjustments may be required to maintain efficacy, but empirical increases are not routinely recommended due to risk of uterine contractions and fetal effects. Plasma protein binding is unchanged.
No specific pharmacokinetic studies in pregnancy; increased renal clearance of dextromethorphan in third trimester may lower exposure. For quinidine, plasma protein binding decreases in pregnancy, potentially increasing free drug fraction. Dose adjustment not recommended due to lack of data; titrate based on clinical response and toxicity monitoring.
NORPACE (disopyramide) is a Vaughan Williams Class Ia antiarrhythmic with negative inotropic effects; avoid in patients with heart failure or reduced LVEF. Monitor QRS and QT intervals; torsades de pointes risk. Adjust dose in renal impairment. Anticholinergic side effects (dry mouth, urinary retention, blurred vision) are common.
Monitor for QT prolongation with baseline and periodic ECG, especially in patients with hypokalemia, hypomagnesemia, or bradycardia. Avoid use with CYP3A4 inhibitors (e.g., ketoconazole) or CYP2D6 inhibitors (e.g., paroxetine) due to quinidine metabolism. Quinidine is a potent CYP2D6 inhibitor; caution with concurrent CYP2D6 substrates like tricyclic antidepressants. Onset of pseudobulbar affect improvement may take weeks; titrate dose gradually.
Take exactly as prescribed; do not miss doses or double up.,Avoid driving if you experience blurred vision or dizziness.,Report chest pain, shortness of breath, fainting, or rapid heartbeat immediately.,May cause dry mouth; sugarless gum or candy can help.,Avoid alcohol and grapefruit juice without consulting your doctor.,Do not stop abruptly; gradual tapering may be needed.
Take this medication exactly as prescribed; do not double doses if missed.,Avoid grapefruit juice as it may increase side effects.,Report any signs of irregular heartbeat (palpitations, dizziness, fainting) or allergic reactions (rash, difficulty breathing).,May cause dizziness or blurred vision; avoid driving until you know how it affects you.,Do not stop abruptly; dose reduction should be gradual under medical supervision.,Inform all healthcare providers about this medication, especially before any surgery or dental procedure.
No interactions on record
"The coadministration of nisoldipine, a calcium channel blocker, with quinidine, a Class Ia antiarrhythmic, results in a significant reduction in quinidine serum concentrations. This interaction is primarily due to nisoldipine-induced enhancement of hepatic quinidine metabolism via cytochrome P450 3A4 induction, leading to subtherapeutic quinidine levels and potential loss of antiarrhythmic efficacy. Clinically, patients may experience breakthrough arrhythmias or inadequate suppression of atrial or ventricular arrhythmias."
"Scopolamine, an anticholinergic agent, and Quinidine, a Vaughan-Williams Class Ia antiarrhythmic with anticholinergic properties, exhibit additive anticholinergic effects when coadministered. This synergy can lead to enhanced peripheral and central anticholinergic adverse effects, including dry mouth, blurred vision, urinary retention, constipation, tachycardia, confusion, and cognitive impairment, particularly in elderly patients. The risk is significant as both drugs block muscarinic acetylcholine receptors, potentially precipitating anticholinergic toxicity."
"Vernakalant, a multi-ion channel blocker primarily used for atrial fibrillation conversion, can inhibit cytochrome P450 (CYP) 3A4, the major enzyme responsible for the metabolism of quinidine. This results in elevated plasma concentrations of quinidine, increasing the risk of quinidine-related cardiotoxicity, including QTc prolongation, torsade de pointes, and other ventricular arrhythmias. The interaction may also potentiate vagolytic effects and negative inotropy, leading to hemodynamic compromise."
Explore head-to-head clinical comparisons of other medications in the same therapeutic classes.
Common clinical questions about NORPACE vs DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE, answered by our medical review team.
NORPACE is a Antiarrhythmic (Class Ia) that works by Class Ic antiarrhythmic agent; blocks voltage-gated sodium channels, slowing conduction velocity and prolonging refractory periods in cardiac tissue.. DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE is a Antiarrhythmic (Class Ia) that works by Dextromethorphan is an uncompetitive NMDA receptor antagonist and sigma-1 receptor agonist; quinidine is a CYP2D6 inhibitor that increases dextromethorphan bioavailability.. They differ in pharmacokinetic profiles, FDA-approved indications, and side effect profiles.
Potency comparisons between NORPACE and DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE depend on the specific clinical indication. These are both Antiarrhythmic (Class Ia) agents and are not directly interchangeable by dose. A physician or clinical pharmacist should guide any therapeutic switching decisions.
The standard adult dose of NORPACE is: 150 mg orally every 6 hours (maximum 300 mg per dose), extended-release formulation 300 mg every 12 hours.. The standard adult dose of DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE is: One capsule (dextromethorphan 20 mg/quinidine 10 mg) orally once daily, with a maximum dose of two capsules per 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 NORPACE and DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE 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. NORPACE is classified as Category C. First trimester: No evidence of increased risk of congenital malformations in human studies, but animal studies are insufficient. Second and third trimesters: Risk of fetal bradyca. DEXTROMETHORPHAN HYDROBROMIDE AND QUINIDINE SULFATE is classified as Category A/B. First trimester: Limited human data; animal studies show quinidine sulfate associated with increased risk of fetal malformations at high doses. Second/third trimester: Dextromethor. Always consult a maternal-fetal medicine specialist before taking either drug during pregnancy or lactation.