Digoxin

Mechanism of Action

Digoxin enhances myocardial contractility through a direct, dose-dependent effect, with measurable activity even at relatively low doses. 

Its primary mechanism is inhibition of the sodium-potassium ATPase (Na⁺/K⁺-ATPase) pump. This inhibition alters transmembrane ionic gradients, increasing Na+ and indirectly increasing intracellular calcium via the sodium-calcium exchange pump. The resulting rise in intracellular calcium enhances excitation-contraction coupling and strengthens cardiac muscle contraction.

The drug’s effects are influenced by serum potassium levels; hypokalaemia potentiates digoxin’s action, while hyperkalaemia reduces its effectiveness. This is due to reduced competition at the K⁺-binding site on the Na⁺/K⁺-ATPase.

In addition to its direct myocardial effects, digoxin acts on the autonomic nervous system by inhibiting Na⁺/K⁺-ATPase in neural tissue. This increases vagal (parasympathetic) tone and reduces sympathetic activity, leading to slowed conduction through the atria and atrioventricular node. Clinically, its most important therapeutic benefit is reduction of ventricular rate, particularly in atrial arrhythmias.

Created in BioRender. Boucher, M. (2026) https://BioRender.com/7csu5zp

Fig 1: Mechanism of action of digoxin

Pharmacokinetics

Digoxin is moderately well absorbed after oral administration, with bioavailability of ~63% (tablets) and ~75% (oral solution). Due to its narrow therapeutic index, dose adjustment is required when switching from oral to IV formulations, with a 33% dose reduction suggested by manufacturers. 

Following intravenous loading, pharmacological effects typically appear within 5–30 minutes. After oral administration, onset of action generally occurs within 30 minutes to 2 hours.

Food slows absorption, and high-fibre meals may reduce the total amount absorbed. It has a large volume of distribution and accumulates extensively in tissues, particularly the heart, while only about 25% is plasma protein bound. Digoxin undergoes minimal metabolism, with most of the drug excreted unchanged by the kidneys. 

Digoxin is a polar drug of which most remains in the renal tubule lumen and is not reabsorbed, meaning that renal elimination is the main factor that determines its duration of action. The terminal half-life in patients with normal renal function is 30–40 hours, whereas in patients with renal impairment it can be up to 100 hours. In clinical terms, this means that digoxin doses should be reduced in renal impairment. Digoxin is a substrate for P-glycoprotein, which influences both intestinal absorption and renal elimination.

Due to its long half life, digoxin requires loading doses in urgent situations to reach therapeutic levels. If a slow digoxin regimen is followed (initiating at a maintenance dose without a loading dose), steady state levels will be achieved after 7-10 days in the average patient (~five times the half life). 

Table 1: Pharmacokinetics of Digoxin

Monitoring

Due to digoxin’s narrow therapeutic index, drug levels are required to be taken. Generally a trough level is taken, 6-24 hours after the last dose of digoxin and 7 days after a dose change or initiation of therapy. In heart failure, a suggested optimum trough level is 0.5-1mcg/L and in AF 0.5-2mcg/L. Toxicity is associated with levels >2mcg/L and likely with levels >3mcg/L

Certain immunoassays (e.g., CMIA) may produce falsely elevated serum digoxin concentrations in patients receiving enzalutamide. Furthermore high doses of biotin (vitamin B7) can lead to falsely elevated digoxin concentrations using the luminescent oxygen channelling immunoassay (LOCI). If results are inconsistent with the clinical picture, confirmation with an alternative assay is recommended.

Adverse Effects

Adverse effects that can occur with digoxin are similar to the signs and symptoms of digoxin toxicity, however in toxicity, effects may be more frequent and severe.

Toxicity 

• Cardiac effects are of the most concern in digoxin toxicity, and can include many types of arrhythmia, which may resemble those that patients are being treated for. 
• Gastrointestinal symptoms include anorexia, nausea, vomiting and abdominal pain.
• Neurological effects include confusion and weakness 
• Visual changes are often associated with toxicity, for example xanthopsia, where objects appear yellow, or blurred vision.
• Electrolyte abnormalities including hyperkalaemia (inhibition of Na⁺/K⁺-ATPase leads to increase in extracellular K⁺) in acute toxicity. Whereas in chronic toxicity, hypokalaemia, hypomagnesemia and hypercalcemia increase the risk of toxic effects of digoxin.

Digoxin toxicity can be treated with the digoxin-specific antibody (DigiFab®) and is indicated for use when there is life threatening digoxin exposure, digoxin level >4ng/ml, digoxin induced hyperkalaemia and digoxin induced bradyarrhythmia. 

Cautions

Use of digoxin requires careful clinical judgement in a number of settings, as its narrow therapeutic index and sensitivity to physiological changes increase the risk of adverse effects.

Digoxin is not contraindicated after myocardial infarction, but caution is advised. Its inotropic effect may increase myocardial oxygen demand and exacerbate ischaemia. Patients may be hypokalaemic or haemodynamically unstable, increasing the risk of arrhythmias. 

The presence of digoxin also limits the safe use of direct current (DC) cardioversion; digoxin is usually withheld 24 hours prior to avoid provoking arrhythmias. In emergencies, the lowest effective energy should be used. Cardioversion is inappropriate for arrhythmias caused by cardiac glycoside toxicity.

Furthermore, digoxin should generally be avoided in heart failure associated with cardiac amyloidosis, myocarditis (due to risk of vasoconstriction), and constrictive pericarditis (unless required for ventricular rate control in atrial fibrillation or systolic dysfunction).
Furthermore, withdrawal of digoxin in stable patients receiving diuretics (with or without ACE inhibitors) may lead to clinical deterioration.

Digoxin may improve exercise tolerance in systolic heart failure, though the benefit of digoxin in patients with supraventricular arrhythmias is more evident at rest, which is why digoxin is often recommended in sedentary patients.

Hypokalaemia significantly increases myocardial sensitivity to digoxin. Hypoxia, hypomagnesaemia, and hypercalcaemia similarly predispose to toxicity. Electrolyte abnormalities should be corrected before and during treatment. Patients with beri-beri heart disease (which occurs due to a lack of thiamine) may not respond to digoxin if they are not treated for the underlying thiamine deficiency.

In addition to this, thyroid status alters digoxin requirements. Reduced doses are needed in hypothyroidism, whereas hyperthyroidism may necessitate higher doses. As thyroid function normalises, digoxin dosage should be adjusted accordingly.

Contraindications

Digoxin is contraindicated in several clinical situations due to the risk of worsening conduction disturbances or precipitating serious arrhythmias.

It should not be used in patients with intermittent complete heart block or second-degree atrioventricular (AV) block, particularly in those with a history of Stokes-Adams attacks. 

Digoxin must be avoided in supraventricular arrhythmias associated with an accessory AV pathway, such as Wolff-Parkinson-White syndrome, unless detailed electrophysiological assessment has excluded harmful effects. If an accessory pathway is known or suspected, even in the absence of prior supraventricular arrhythmias, digoxin should not be administered.

Additional contraindications include ventricular tachycardia, ventricular fibrillation, and hypertrophic obstructive cardiomyopathy (except in carefully selected cases with coexisting atrial fibrillation and heart failure, where extreme caution is required). Finally, digoxin is contraindicated in patients with hypersensitivity to digoxin, other digitalis glycosides, or any of its excipients.

Interactions

P-glycoprotein Inhibitors 

Digoxin is a P-glycoprotein (P-gp) efflux pump substrate which leads to many clinically significant interactions. Inhibition of the P-gp efflux pump may lead to increased digoxin absorption in the gut and reduced renal clearance, overall leading to an increase in exposure to digoxin. In this case, reduced digoxin doses may be required. Conversely P-gp inducers may reduce digoxin levels and increased digoxin doses may be required to achieve a therapeutic response. 

Table 2: P-gp inhibitors and inducers

Drugs that cause hypokalaemia may increase the risk of toxicity to digoxin, these include; lithium, corticosteroids, loop and thiazide like diuretics. Diuretics, even those that do not cause hypokalaemia (i.e. spironolactone), may also increase risk of digoxin toxicity due to a decrease in the glomerular filtration rate.

Tetracycline and erythromycin can also interfere with the metabolism of digoxin (specifically the hydrolysis pathway) leading to increased digoxin levels in ~10% of patients, where this pathway is significant for the metabolism of digoxin. 

Colestyramine, sucralfate and antacids bind to digoxin preventing its intestinal absorption, therefore administration should be separated.

Finally, quinidine, verapamil and amiodarone displace digoxin from tissue-binding sites and also inhibit renal tubular secretion of digoxin, this can lead to digoxin toxicity. If verapamil and digoxin are used together, the dose of digoxin can be reduced to avoid toxicity alongside frequent monitoring.

References

1. Digoxin 0.125 mg Tablets – Summary of Product Characteristics (SmPC) – (emc) | 5461 accessed 24/02/26
2. Inhibitors and inducers of P-glycoprotein drug efflux – UpToDate Accessed 25/02/26
3. Ritter JM, Flower RJ, Henderson G, Loke YK, MacEwan DJ. Rang & Dale’s Pharmacology. 9th ed. London: Elsevier; 2019. 
4. https://www.uptodate.com/contents/treatment-with-digoxin-initial-dosing-monitoring-and-dose-modification?search=digoxin%20interactions&source=search_result&selectedTitle=3~150&usage_type=default&display_rank=2 Accessed 25/02/26
5. https://www.uptodate.com/contents/digitalis-cardiac-glycoside-poisoning?search=digoxin%20toxicity&source=search_result&selectedTitle=1~70&usage_type=default&display_rank=1 Accessed 25/02/26
6. https://cks.nice.org.uk/topics/atrial-fibrillation/prescribing-information/digoxin/ Accessed 26/02/26