Drug Reference
Drug Reference

Cardiac Drug Interactions

A clinically focused guide to high-risk drug-drug interactions in cardiology and electrophysiology — QT prolongation, antiarrhythmic combinations, DOAC pitfalls, and statin safety

QT Prolongation Antiarrhythmics DOACs Statins Device Interactions
Updated: March 2026
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QT-Prolonging Drug Combinations

Drug-induced QT prolongation is one of the most common preventable causes of torsades de pointes (TdP) and sudden cardiac death. The risk is dramatically amplified when two or more QT-prolonging drugs are combined. Understanding which medications carry the highest risk, and which patient-level factors compound that risk, is essential for every clinician managing cardiac patients.

High-Risk Combination Categories

The most dangerous drug-drug interactions for QT prolongation occur when antiarrhythmic agents (which inherently prolong the QT interval) are co-administered with other QT-prolonging medications from different classes:

High-Risk QT-Prolonging Drug Combinations
Drug Class Specific Agents Mechanism Risk Level
Antibiotics Azithromycin, levofloxacin, moxifloxacin, TMP-SMX, erythromycin hERG (IKr) channel blockade High (especially with Class III AADs)
Antifungals Fluconazole, voriconazole, ketoconazole hERG blockade + CYP3A4 inhibition (increased AAD levels) High
Antipsychotics Haloperidol, quetiapine, ziprasidone, thioridazine hERG blockade (dose-dependent) High (IV haloperidol highest)
Antiemetics Ondansetron (IV), droperidol, domperidone hERG blockade Moderate-High
Opioids Methadone hERG blockade (dose-dependent, significant >100 mg/day) High
Antidepressants Citalopram, escitalopram (dose-dependent) hERG blockade Moderate (dose caps: citalopram ≤40 mg)

Modifiable Risk Factors for TdP

The Tisdale Risk Score identifies patients at highest risk for QT prolongation during hospitalization. Modifiable risk factors that must be corrected before and during QT-prolonging drug therapy include:

  • Hypokalemia: Maintain K+ ≥ 4.0 mEq/L (reduces IKr repolarization reserve)
  • Hypomagnesemia: Maintain Mg2+ ≥ 2.0 mg/dL (IV magnesium is first-line for TdP)
  • Bradycardia: Pause-dependent TdP; consider isoproterenol or temporary pacing
  • Female sex: Women have ~20 ms longer baseline QTc; 2-3x higher TdP risk
  • Baseline QTc >500 ms: Exponential risk increase above this threshold
  • Recent cardioversion: Transient QT prolongation post-cardioversion
  • Heart failure / reduced LVEF: Reduced repolarization reserve
  • Hepatic or renal impairment: Impaired drug metabolism/excretion

QT-Prolonging Drugs by Risk Category (CredibleMeds Classification)

CredibleMeds.org maintains the most authoritative, evidence-based classification of QT-prolonging drugs. The following table summarizes cardiovascular-relevant agents:

QT-Prolonging Drugs by CredibleMeds Risk Category
Known Risk (TdP documented) Possible Risk (QT prolongation, limited TdP data) Conditional Risk (TdP with specific conditions)
Amiodarone Azithromycin Amitriptyline (overdose)
Dofetilide Quetiapine Diphenhydramine (overdose)
Sotalol Ranolazine Fluoxetine (overdose, hypokalemia)
Droperidol Trazodone Metformin (with renal failure)
Haloperidol (IV) Granisetron Furosemide (via hypokalemia)
Methadone Famotidine HCTZ (via hypokalemia)
Ondansetron (IV) Lithium Indapamide (via hypokalemia)
Erythromycin Risperidone Loperamide (abuse/supratherapeutic)
Fluconazole Rilpivirine Donepezil (with bradycardia)
Moxifloxacin Tamoxifen Voriconazole (with electrolyte abnormalities)
Citalopram / Escitalopram Atazanavir Ibuprofen (overdose)
Chlorpromazine Ciprofloxacin Solifenacin (with CYP3A4 inhibitors)
Procainamide Clozapine Tacrolimus (with hypokalemia)
Quinidine Felbamate Trimethoprim (via hyperkalemia in context of dofetilide)
Ibutilide Fingolimod Chloroquine (overdose)
Clinical Pearl: The most dangerous period for drug-induced TdP is during drug initiation or dose escalation, not during chronic steady-state therapy. For dofetilide and sotalol, this is why in-hospital ECG monitoring for a minimum of 3 days (dofetilide) or 3 doses (sotalol) is mandatory. Always check a QTc 2-3 hours after the second or third dose, as this captures peak drug effect. If QTc exceeds 500 ms (or increases by >60 ms from baseline), the drug should be dose-reduced or discontinued.
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Antiarrhythmic Drug Interactions

Antiarrhythmic drugs (AADs) have narrow therapeutic windows and complex pharmacokinetics, making them among the most interaction-prone medications in cardiology. Most serious interactions involve either pharmacokinetic mechanisms (CYP450 inhibition, P-glycoprotein modulation) or pharmacodynamic mechanisms (additive QT prolongation, enhanced negative inotropy/chronotropy).

Amiodarone Interactions

Amiodarone is a potent inhibitor of CYP2C9, CYP2D6, CYP3A4, and P-glycoprotein (P-gp). Its exceptionally long half-life (40-55 days) means drug interactions can persist for weeks to months after discontinuation.

Amiodarone Drug Interactions
Interacting Drug Mechanism Clinical Effect Required Action
Warfarin CYP2C9 inhibition INR increases ~50% (range 25-65%) Reduce warfarin dose by 30-50%; check INR within 1 week
Digoxin P-gp inhibition + renal clearance reduction Digoxin level increases ~70-100% Reduce digoxin dose by 50%; monitor levels
Simvastatin CYP3A4 inhibition Rhabdomyolysis risk Simvastatin max 20 mg/day; consider rosuvastatin or pravastatin
Lovastatin / Atorvastatin CYP3A4 inhibition Increased statin exposure; myopathy risk Use lowest effective dose; prefer rosuvastatin or pravastatin
Dabigatran P-gp inhibition Increased dabigatran levels ~12-60% Avoid if CrCl <30; no dose adjustment if CrCl ≥30
Rivaroxaban / Apixaban P-gp inhibition (mild CYP3A4 component) Modestly increased DOAC levels Use with caution; monitor for bleeding
Phenytoin / Fosphenytoin CYP2C9 inhibition (bidirectional: phenytoin induces amiodarone metabolism) Increased phenytoin toxicity; decreased amiodarone efficacy Monitor phenytoin levels; consider alternative anticonvulsant
Cyclosporine CYP3A4 + P-gp inhibition Increased cyclosporine levels; nephrotoxicity Reduce cyclosporine dose; monitor trough levels

Flecainide & Propafenone + CYP2D6 Inhibitors

Both flecainide and propafenone are extensively metabolized by CYP2D6. Approximately 7-10% of Caucasians are CYP2D6 poor metabolizers at baseline. Co-administration with CYP2D6 inhibitors effectively converts extensive metabolizers into poor metabolizers, dramatically increasing drug levels:

  • Fluoxetine — potent CYP2D6 inhibitor; can double flecainide/propafenone levels
  • Paroxetine — potent CYP2D6 inhibitor; same risk as fluoxetine
  • Bupropion — moderate CYP2D6 inhibitor; clinically significant interaction
  • Quinidine — potent CYP2D6 inhibitor; also adds QT prolongation
  • Terbinafine — potent CYP2D6 inhibitor; often overlooked

Sotalol Interactions

Sotalol combines Class III (IKr blockade) with non-selective beta-blockade. The QT prolongation is dose-dependent and renally cleared. Key interactions:

  • Other QT-prolonging agents: Contraindicated — additive TdP risk is synergistic, not merely additive
  • Drugs causing hypokalemia/hypomagnesemia: Loop diuretics, thiazides, amphotericin B — correct electrolytes aggressively
  • Clonidine: Rebound hypertension risk upon clonidine discontinuation is amplified
  • Antacids (aluminum/magnesium hydroxide): Reduce sotalol absorption by up to 25%; separate dosing by 2 hours

Dofetilide — Absolute Contraindications

Dofetilide has the most restrictive drug interaction profile of any AAD. The following drugs are absolutely contraindicated because they inhibit the renal cation transporter (OCT2) responsible for dofetilide elimination, causing potentially fatal accumulation:

Dofetilide Absolute Contraindications
Contraindicated Drug Mechanism Clinical Consequence
Verapamil OCT2 inhibition + additive AV nodal suppression Markedly increased dofetilide levels; TdP
Cimetidine OCT2 inhibition (potent) Dofetilide AUC increases ~50%; TdP
Hydrochlorothiazide (HCTZ) OCT2 inhibition + hypokalemia Dual mechanism for TdP
Ketoconazole CYP3A4 inhibition + OCT2 inhibition Increased dofetilide levels; TdP
Trimethoprim (alone or as TMP-SMX) OCT2 inhibition Dofetilide AUC increases ~100%; TdP
Megestrol OCT2 inhibition Increased dofetilide levels; TdP
Prochlorperazine OCT2 inhibition + additive QT prolongation Increased dofetilide levels; TdP
Dolutegravir OCT2 inhibition (potent) Significantly increased dofetilide levels; TdP

Dronedarone Interactions

Dronedarone is a moderate inhibitor of CYP3A4 and P-gp. Key interactions include:

  • Digoxin: P-gp inhibition increases digoxin levels ~2.5-fold; reduce digoxin dose by 50% and monitor levels
  • Dabigatran: P-gp inhibition increases dabigatran exposure; reduce dabigatran to 75 mg BID (if CrCl 30-50) or avoid
  • Simvastatin/Lovastatin: CYP3A4 inhibition increases statin levels 2-4x; limit simvastatin to 10 mg/day
  • Calcium channel blockers (diltiazem, verapamil): Additive AV nodal blockade + CYP3A4 interaction; use with extreme caution at low CCB doses
  • Strong CYP3A4 inhibitors (ketoconazole, ritonavir): Contraindicated — markedly increase dronedarone levels
Clinical Pearl: When starting amiodarone in a patient on warfarin, proactively reduce the warfarin dose by 30-50% at the time of amiodarone initiation. Do not wait for the INR to rise. Due to amiodarone's long loading phase, the INR elevation may not manifest for 1-2 weeks but can be severe. Check the INR within 5-7 days and weekly for the first month.
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DOAC Drug Interactions

All four DOACs (dabigatran, rivaroxaban, apixaban, edoxaban) are substrates of P-glycoprotein (P-gp), and rivaroxaban and apixaban are also metabolized by CYP3A4. Drug interactions that modulate these pathways can significantly increase bleeding risk (with inhibitors) or reduce efficacy and increase stroke risk (with inducers).

CRITICAL — DOAC INTERACTION PRINCIPLES
  • Dual P-gp + CYP3A4 inhibitors (ketoconazole, itraconazole, ritonavir, clarithromycin) are CONTRAINDICATED with all DOACs due to dramatically increased drug exposure and bleeding risk
  • P-gp/CYP3A4 inducers (rifampin, phenytoin, carbamazepine, phenobarbital, St. John's wort) should be AVOIDED with all DOACs — they can reduce DOAC levels by >50%, leading to therapeutic failure and stroke
  • Single-pathway inhibitors (P-gp only) require dose adjustment depending on the specific DOAC and the patient's renal function

DOAC Interaction Reference Table

DOAC Drug Interactions — Action Required by Drug and Pathway
Interacting Drug Pathway Dabigatran Rivaroxaban Apixaban Edoxaban
Ketoconazole / Itraconazole P-gp + CYP3A4 CONTRAINDICATED CONTRAINDICATED CONTRAINDICATED (or reduce to 2.5 mg BID) CONTRAINDICATED
Ritonavir / HIV PIs P-gp + CYP3A4 CONTRAINDICATED AVOID AVOID AVOID
Clarithromycin P-gp + CYP3A4 CONTRAINDICATED if CrCl <30 AVOID Reduce dose if ≥2 dose-reduction criteria met AVOID
Amiodarone P-gp Caution; avoid if CrCl <30 No adjustment No adjustment No adjustment
Dronedarone P-gp + moderate CYP3A4 CONTRAINDICATED Caution Caution Reduce to 30 mg daily
Verapamil P-gp Reduce to 110 mg BID (or 75 mg BID if CrCl 30-50) No adjustment No adjustment No adjustment
Diltiazem P-gp + weak CYP3A4 Caution No adjustment No adjustment No adjustment
Quinidine P-gp CONTRAINDICATED No adjustment No adjustment Reduce to 30 mg daily
Cyclosporine P-gp + CYP3A4 CONTRAINDICATED AVOID AVOID AVOID
Tacrolimus P-gp Caution; monitor Caution; monitor Caution; monitor Caution; monitor
Rifampin P-gp + CYP3A4 inducer AVOID (reduces levels >65%) AVOID (reduces levels ~50%) AVOID (reduces levels ~55%) AVOID
Phenytoin / Carbamazepine P-gp + CYP3A4 inducer AVOID AVOID AVOID AVOID
St. John's Wort P-gp + CYP3A4 inducer AVOID AVOID AVOID AVOID
Clinical Pearl: For patients on dabigatran + verapamil, the verapamil must be taken at least 2 hours after the dabigatran to minimize interaction magnitude. This timing-dependent interaction is unique to dabigatran because verapamil inhibits intestinal P-gp, increasing dabigatran absorption. If co-administered simultaneously, dabigatran AUC increases by ~180%; if separated by 2 hours, the increase is only ~50%.
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Statin & Cardiovascular Drug Interactions

Statins are among the most commonly prescribed medications in cardiovascular patients. Simvastatin, atorvastatin, and lovastatin are metabolized by CYP3A4, making them vulnerable to drug interactions that increase the risk of rhabdomyolysis. Rosuvastatin, pravastatin, and pitavastatin are largely CYP-independent and therefore safer in polypharmacy settings.

CYP3A4-Metabolized Statin Interactions

Statin Dose Limits with CYP3A4 Inhibitors
CYP3A4 Inhibitor Simvastatin Lovastatin Atorvastatin Recommended Alternative
Amiodarone Max 20 mg/day Max 40 mg/day Caution at high doses Rosuvastatin, pravastatin
Verapamil Max 20 mg/day Max 20 mg/day Caution at high doses Rosuvastatin, pravastatin
Diltiazem Max 20 mg/day Max 20 mg/day Caution at high doses Rosuvastatin, pravastatin
Amlodipine Max 20 mg/day Max 20 mg/day No specific cap Rosuvastatin, pravastatin
Clarithromycin / Erythromycin CONTRAINDICATED CONTRAINDICATED AVOID or use lowest dose Rosuvastatin, pravastatin; use azithromycin instead
Itraconazole / Ketoconazole CONTRAINDICATED CONTRAINDICATED AVOID Rosuvastatin, pravastatin
HIV Protease Inhibitors CONTRAINDICATED CONTRAINDICATED Max 20 mg/day (tipranavir/ritonavir: AVOID) Rosuvastatin (max 10 mg), pravastatin, pitavastatin
Cyclosporine CONTRAINDICATED CONTRAINDICATED Max 10 mg/day Pravastatin (max 20 mg), pitavastatin (max 2 mg)
Grapefruit juice (>1 quart/day) AVOID large quantities AVOID large quantities Limit consumption Rosuvastatin, pravastatin (no interaction)
Dronedarone Max 10 mg/day Max 20 mg/day Caution at high doses Rosuvastatin, pravastatin
Ranolazine Max 20 mg/day Max 20 mg/day No specific cap Rosuvastatin, pravastatin

Other Critical Cardiovascular Drug Interactions

Additional High-Risk Cardiovascular Drug Interactions
Drug Combination Risk Mechanism Clinical Action
ARNI (sacubitril/valsartan) + ACE inhibitor Life-threatening angioedema Dual neprilysin + ACE inhibition causes excessive bradykinin accumulation Mandatory 36-hour washout between last ACEi dose and first ARNI dose
ACEi/ARB + MRA + NSAID ("Triple Whammy") Acute kidney injury + hyperkalemia NSAIDs block compensatory prostaglandin-mediated renal blood flow; additive anti-aldosterone effect Avoid triple combination; if NSAID necessary, use shortest duration and monitor Cr/K+ within 48-72 hours
Digoxin + amiodarone Digoxin toxicity P-gp inhibition + reduced renal clearance Reduce digoxin dose by 50%; target level 0.5-0.9 ng/mL
Digoxin + verapamil Digoxin toxicity + AV block P-gp inhibition + additive AV nodal blockade Reduce digoxin dose by 50%; monitor for bradycardia
Digoxin + quinidine Digoxin toxicity P-gp inhibition (potent); displaces tissue binding Reduce digoxin dose by 50%; monitor levels
Ranolazine + strong CYP3A4 inhibitors Ranolazine toxicity (QT prolongation, dizziness, nausea) CYP3A4 inhibition increases ranolazine levels >3-fold Ketoconazole, itraconazole, clarithromycin, ritonavir: CONTRAINDICATED. Diltiazem/verapamil: limit ranolazine to 500 mg BID
Clinical Pearl: When a patient on a CYP3A4-metabolized statin requires a macrolide antibiotic, the safest choice is azithromycin (minimal CYP3A4 inhibition) rather than clarithromycin or erythromycin. If clarithromycin is necessary (e.g., H. pylori triple therapy), temporarily hold the statin for the duration of antibiotic therapy. The rhabdomyolysis risk from even a short course of a strong CYP3A4 inhibitor with simvastatin or lovastatin is clinically significant.
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Device & Procedural Drug Interactions

Drug interactions extend beyond pharmacokinetics and pharmacodynamics into the realm of cardiac devices and procedural medicine. Understanding how specific medications affect device function, MRI safety, and procedural outcomes is critical for the practicing cardiologist and electrophysiologist.

Dronedarone & ICD Function

Dronedarone can increase the defibrillation threshold (DFT) in patients with implantable cardioverter-defibrillators (ICDs). This means the energy required to successfully terminate ventricular fibrillation increases, potentially rendering the ICD's maximum energy output insufficient. After dronedarone initiation in ICD patients, DFT testing should be considered to ensure an adequate safety margin. Additionally, dronedarone may alter pacing thresholds and sensing parameters, requiring device re-interrogation within 1-2 weeks of initiation.

MRI & Drug Interactions

Specific drug formulations require attention in the MRI environment:

  • Transdermal patches with metallic backing: Fentanyl patches, nicotine patches, clonidine patches, and certain nitroglycerin patches contain aluminum or other metallic components that can cause burns during MRI. All transdermal patches should be removed before MRI scanning and replaced afterward
  • Drug infusion pumps: Implantable insulin pumps and intrathecal baclofen pumps may malfunction in the MRI magnetic field
  • Ferromagnetic drug components: Some compounded medications and certain capsule formulations contain iron oxide colorants that can cause artifact

Antithrombotic Combinations — Triple Therapy

Patients requiring both anticoagulation (for atrial fibrillation) and dual antiplatelet therapy (after PCI/stenting) face substantially increased bleeding risk with "triple therapy." Key trials guiding duration minimization:

Landmark Trials for Antithrombotic Combination Strategies
Trial Year Key Finding Clinical Implication
WOEST 2013 VKA + clopidogrel (no aspirin) reduced bleeding without increasing thrombotic events vs. triple therapy First evidence supporting dual pathway (dropping aspirin) over triple therapy
AUGUSTUS 2019 Apixaban + P2Y12 inhibitor (without aspirin) reduced bleeding vs. VKA-based and aspirin-containing regimens Supports DOAC-based dual therapy; aspirin increases bleeding without clear ischemic benefit
ENTRUST-AF PCI 2019 Edoxaban + P2Y12 inhibitor was noninferior for bleeding vs. VKA-based triple therapy Supports early transition to dual therapy with edoxaban
RE-DUAL PCI 2017 Dabigatran + P2Y12 inhibitor reduced bleeding vs. VKA triple therapy Dabigatran 110 mg or 150 mg BID + P2Y12 inhibitor as dual therapy option
PIONEER AF-PCI 2016 Low-dose rivaroxaban strategies reduced bleeding vs. VKA triple therapy First DOAC trial in AF + PCI patients; rivaroxaban 15 mg daily + P2Y12 inhibitor
CURRENT RECOMMENDATIONS — ANTITHROMBOTIC STRATEGY AFTER PCI IN AF
  • Default strategy: DOAC + P2Y12 inhibitor (clopidogrel preferred) — no aspirin beyond 1 week peri-PCI
  • High ischemic risk (e.g., left main PCI, bifurcation stenting, prior stent thrombosis): Consider extending triple therapy to 1 month, then drop aspirin
  • High bleeding risk (HAS-BLED ≥3): Transition to dual therapy immediately after PCI or within 1 week
  • At 12 months: Discontinue antiplatelet; continue DOAC monotherapy for stroke prevention
  • Procedural sedation: Propofol combined with amiodarone can cause enhanced hypotension due to additive vasodilation and negative inotropy; use reduced propofol doses and have vasopressors available

Key References

  1. Woosley RL, Heise CW, Gallo T, Tate J, Woosley D, Romero KA. CredibleMeds.org — QTdrugs List. AZCERT, Inc. Accessed 2026. https://crediblemeds.org
  2. Tisdale JE, Jaynes HA, Kingery JR, et al. Development and validation of a risk score to predict QT interval prolongation in hospitalized patients. Circ Cardiovasc Qual Outcomes. 2013;6(4):479-487. DOI: 10.1161/CIRCOUTCOMES.113.000152
  3. Lopes RD, Heizer G, Aronson R, et al. Antithrombotic therapy after acute coronary syndrome or PCI in atrial fibrillation (AUGUSTUS). N Engl J Med. 2019;380(16):1509-1524. DOI: 10.1056/NEJMoa1817083
  4. Wiggins BS, Dixon DL, Neyber RD, et al. Select drug-drug interactions with direct oral anticoagulants: JACC review topic of the week. J Am Coll Cardiol. 2020;75(11):1341-1350. DOI: 10.1016/j.jacc.2019.12.068
  5. Fenner KS, Troutman MD, Kempshall S, et al. Drug-drug interactions mediated through P-glycoprotein: clinical relevance and in vitro-in vivo correlation using digoxin as a probe drug. Clin Pharmacol Ther. 2009;85(2):173-181. DOI: 10.1038/clpt.2008.195