Digoxin and amiodarone should not be used together without careful dose adjustment and close clinical monitoring—amiodarone significantly increases digoxin blood levels, raising the risk of digoxin toxicity including life-threatening heart rhythm disturbances. The FDA labeling for amiodarone explicitly warns of this interaction, and clinical data suggest that patients on this combination require a 50% reduction in digoxin dose or therapeutic drug monitoring to prevent adverse events.
The FDA drug labeling for amiodarone (Cordarone) carries explicit warnings about digoxin co-administration. The amiodarone prescribing information states that "amiodarone inhibits the metabolism of digoxin, causing an increase in serum digoxin concentration," and recommends that when the two drugs are used together, "the digoxin dose should be reduced by 50% or serum digoxin levels monitored." The FDA classifies this as a significant drug interaction requiring intervention.
Digoxin labeling similarly notes that certain antiarrhythmic drugs, including amiodarone, increase digoxin serum levels and risk of toxicity. Notably, digoxin has a narrow therapeutic index—the margin between an effective dose and a toxic dose is small, typically 0.5–2.0 ng/mL for serum concentration. Even modest increases in digoxin levels can tip a patient into the toxic range, where serious arrhythmias and conduction abnormalities emerge.
The FDA Adverse Event Reporting System (FAERS) has documented cases of digoxin toxicity in patients concurrently receiving amiodarone, though the exact frequency is not publicly stratified in a way that allows precise incidence estimation from publicly available data. However, clinical pharmacology literature and retrospective cohort studies suggest that unmanaged digoxin-amiodarone combinations result in digoxin toxicity in a meaningful proportion of co-exposed patients if doses are not adjusted.
The digoxin-amiodarone interaction operates through multiple pharmacological mechanisms, all of which converge on increasing digoxin exposure.
Renal Clearance Inhibition: Digoxin is eliminated primarily via renal excretion—approximately 70% of an administered dose is excreted unchanged in the urine. Amiodarone and its active metabolite desethylamiodarone inhibit renal tubular secretion of digoxin via organic cation transporters (OCT2) in the proximal tubule. This reduces the net renal clearance of digoxin, causing accumulation over time. Studies have shown that amiodarone reduces digoxin renal clearance by 25–50%, depending on individual variation in transporter expression and amiodarone dosing.
Protein Binding Competition: Both digoxin and amiodarone are highly bound to plasma and tissue proteins. Amiodarone may displace digoxin from protein binding sites, transiently increasing the free (unbound) fraction of digoxin available to reach cardiac tissue—the site of both therapeutic and toxic action. This effect is modest but can contribute to acute increases in digoxin effect shortly after amiodarone initiation.
Tissue Distribution and P-glycoprotein Inhibition: Digoxin is a substrate for P-glycoprotein (MDR1), an efflux transporter found in the intestinal epithelium, renal tubules, and cardiac myocytes. Amiodarone inhibits P-glycoprotein function, reducing the outward extrusion of digoxin from cells. This leads to increased intracellular digoxin concentration in cardiac tissue, amplifying both efficacy and toxicity at the Na-K-ATPase pump—digoxin's site of action.
Cumulative Mechanism: Because digoxin has a long half-life (36–40 hours in patients with normal renal function, often much longer in the elderly or those with renal impairment) and a narrow therapeutic index, even a 25–50% reduction in clearance causes substantial accumulation. The clinical effect is often delayed by several days, making early recognition of the problem challenging.
Age and Renal Function: Elderly patients (age ≥75 years) are at heightened risk because they have age-related declines in glomerular filtration rate (GFR) and baseline renal tubular function. Digoxin clearance in an 80-year-old may be half that of a 40-year-old. Amiodarone's inhibition of digoxin renal clearance compounds this decline. Any patient with a baseline GFR <60 mL/min/1.73m² is at increased risk.
Body Weight and Lean Body Mass: Digoxin dosing is traditionally weight-based, and underweight or elderly patients with low lean body mass often receive inappropriately high doses relative to their body composition. When amiodarone is then added, these patients are already at the upper end of their safe digoxin concentration range.
Drug History and Polypharmacy: Patients taking other medications that inhibit renal function or compete for renal secretion—such as NSAIDs, ACE inhibitors, or diuretics that cause volume depletion—face cumulative risk. Similarly, patients on other antiarrhythmics (verapamil, diltiazem) have additive risks.
Cardiac Status: Patients with heart failure, hypokalemia, or hypercalcemia are more susceptible to digoxin toxicity even at "normal" serum levels. These are exactly the patient populations that often require both digoxin (for rate control and contractility in heart failure) and amiodarone (for arrhythmia suppression).
A 78-year-old man with a 10-year history of atrial fibrillation and systolic heart failure (ejection fraction 35%) has been stable on digoxin 0.25 mg daily for 8 years. His serum digoxin concentration is 1.2 ng/mL (therapeutic range), and his renal function is borderline: creatinine 1.4 mg/dL, estimated GFR 48 mL/min/1.73m². He now presents with recurrent rapid atrial fibrillation (ventricular rate 110–130 bpm despite the digoxin) and is started on amiodarone 400 mg daily for 7 days, then 200 mg daily maintenance.
Within 10 days, he develops nausea, fatigue, and palpitations. An ECG shows new frequent premature ventricular contractions and a run of non-sustained ventricular tachycardia. A stat digoxin level is 2.8 ng/mL—well into the toxic range. He is admitted to telemetry, his digoxin is held, and electrolytes are checked (potassium 3.2 mEq/L, low). After electrolyte repletion and 48 hours off digoxin, his arrhythmia resolves. He is restarted at digoxin 0.125 mg daily (50% reduction) with amiodarone continued, and a repeat level 5 days later is 1.4 ng/mL—therapeutic and safe.
Key Learning: The combination triggered toxicity because amiodarone reduced his already-compromised renal digoxin clearance by an estimated 30–40%. His serum concentration nearly doubled within 10 days. Early recognition was difficult because his symptoms (nausea, fatigue) could be attributed to the arrhythmia itself. Had his digoxin dose not been reduced proactively or monitored, he was at risk for life-threatening arrhythmia.
A 65-year-old woman with a history of hypertrophic cardiomyopathy (HCM) is hospitalized for syncope. She has been on digoxin 0.25 mg twice daily for 15 years (a higher-than-modern dosing, but established in her case) and metoprolol. Holter monitoring reveals sustained ventricular tachycardia. The cardiologist initiates amiodarone 150 mg IV loading dose, then 1 mg/min infusion for 6 hours, followed by an oral amiodarone plan (400 mg daily).
On hospital day 2, routine labs show a serum digoxin concentration of 2.2 ng/mL (up from a baseline of 1.5 ng/mL just before admission). The patient develops subtle cognitive changes (confusion, mild disorientation) and complains of blurred vision. These are classic signs of digoxin toxicity. The team recognizes the interaction: amiodarone, even IV and just started, is already inhibiting digoxin clearance. Her digoxin is immediately discontinued, and the amiodarone is continued at a lower maintenance dose (200 mg daily). Serum potassium and magnesium are optimized (digoxin toxicity risk rises sharply with hypokalemia). Within 3 days, her mental status clears, and her digoxin level drifts down to 1.1 ng/mL. She is discharged on amiodarone 200 mg daily without digoxin—amiodarone alone adequately controls her rate and arrhythmia.
Key Learning: Even IV amiodarone, administered acutely, can trigger the interaction within hours to days. The cognitive and visual symptoms are hallmark digoxin toxicity signs that can be mistaken for other causes (stroke, delirium from hospitalization). Vigilance and a low threshold for checking digoxin levels in this population are critical.
For Patients Already on Digoxin Who Are Starting Amiodarone:
For Patients Starting Both Digoxin and Amiodarone Together:
For Patients on Amiodarone Who Need Digoxin Later:
Contact your healthcare provider immediately if you experience any of the following while taking digoxin and amiodarone together:
Do not wait for a scheduled appointment if these symptoms emerge. Call your pharmacist or doctor the same day, or go to an urgent care or emergency department if symptoms are severe or accompanied by chest pain or fainting.
If you are taking digoxin and amiodarone—or any combination of medications—verify your complete drug interaction profile today. Use checkdruginteractions.com, the most comprehensive drug interaction checker powered by over 250,000 FDA drug labels. Enter your full medication list to uncover hidden interactions before they become clinical problems. Your pharmacist and doctor should review your results, but this tool gives you and your caregivers the data-driven insight you need to stay safe.
CDI checks every pair across up to 20 drugs — backed by FDA and NIH data.
Drug interaction data sourced from U.S. FDA drug labeling via openFDA and the U.S. National Library of Medicine (NLM), National Institutes of Health. For informational purposes only. Always consult your pharmacist or physician before making any medication decisions.
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