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Digoxin is a prescription cardiac medication used for horses in selected cases of congestive heart failure and certain abnormal heart rhythms, especially atrial fibrillation. Because it affects both the strength of heart contractions and the way electrical signals move through the heart, its use requires careful diagnosis and close veterinary supervision.
While this medication can play a helpful role in some cardiovascular cases, it is not a routine treatment and is generally reserved for situations where the potential benefits outweigh the risks.
One of the most important things to know about digoxin is that it has a narrow margin of safety. Small changes in dose, hydration status, kidney function, electrolyte balance, or other medications can affect how the drug works and increase the risk of adverse effects, making monitoring a key part of treatment.
Keep reading to learn how digoxin works in horses, when veterinarians may use it, and the safety considerations, side effects, and monitoring needs for safe and effective treatment with this medication.
Digoxin for Horses
Digoxin is a medication used in equine medicine to help manage certain heart conditions, particularly congestive heart failure and abnormal heart rhythms such as atrial fibrillation. [1][2][3]
Digoxin is derived from plants in the Digitalis genus, commonly known as foxglove. It is a positive inotrope, meaning it increases the strength of heart contractions. It can also help slow heart rate by affecting how electrical signals move through the heart. [1][4]
Digoxin must be used carefully because it has a narrow margin of safety, which means small dosing changes can lead to toxicity. As a result, horses receiving digoxin often require close veterinary monitoring and dose adjustments during treatment. [2][5][6]
In horses, there is considerable variation in how quickly the drug is metabolized and eliminated from the body. This may affect how well the medication works and increase the risk of side effects in some cases. [2][5][6]
Due to these risks and the availability of newer therapies, digoxin is currently used less often in equine medicine than it has been in historical practice. [2]
There are no formulations of digoxin approved specifically for equine use, which means it is prescribed to horses in an off-label (also referred to as extra-label) manner.
Available Forms
Because there are no FDA-approved digoxin formulations specifically labeled for horses, veterinarians rely on off-label use of human-grade medications when prescribing this medication to horses.
Available forms include: [2]
- Oral tablets
- Oral solution
- Injectable formulations for intravenous administration
Alongside existing approval for its use in humans, there is some clinical data supporting digoxin use in horses. [1][5][6] Veterinarians use this information and their professional expertise to make dosing and treatment decisions involving digoxin for horses, which is permitted under veterinary prescribing regulations.
What's your top priority with your horse's health?
What's your top priority with your horse's health?
Drug Class: Cardiac Glycosides
Cardiac glycosides are compounds that increase the force of heart muscle contractions while also affecting electrical conduction within the heart. They produce these effects mainly by blocking sodium-potassium ATPase pumps in heart muscle cells. [1]
Digitoxin is a drug in this class. [1]
Mechanism of Action
Normal cardiac function in horses depends on coordinated electrical activity and effective movement of blood through the heart. [7][8] Electrical impulses normally originate in the sinoatrial node, spread through the atria, pass through the atrioventricular (AV) node, and then stimulate ventricular contraction. [8]
The autonomic nervous system helps regulate heart rhythm and conduction. [8] Horses are particularly predisposed to certain arrhythmias because they have large atrial muscle mass and naturally high vagus nerve tone. [9]
Cardiac arrhythmias occur when normal electrical conduction is disrupted. [10][11] Atrial fibrillation (AF) is one of the most common arrhythmias associated with poor performance in athletic horses. [12]
Illustration: Dr. Madison Ricard, DVM, PhD, DACVP, PAS
In horses with AF, the atria no longer contract in a coordinated manner and instead produce multiple irregular electrical impulses. [12][13] This results in an irregular heartbeat and reduced cardiac efficiency, particularly during exercise. [9]
Horses are especially prone to AF because their large atrial size favors development of electrical circuits within the atrial tissue. [9][12][13][14]
Illustration: Dr. Madison Ricard, DVM, PhD, DACVP, PAS
Digoxin works by blocking sodium-potassium ATPase pumps on heart muscle cells. This causes sodium levels inside the cells to rise, impacting calcium exchange across muscle cell membranes, which is fundamental to muscle contraction. As a result, more calcium remains available inside heart muscle cells, resulting in more powerful heart muscle contractions. [1]
In addition to increasing the strength of heart contractions, digoxin also affects the nervous system signals that help control heart function. It increases parasympathetic (vagal) activity and decreases sympathetic activity, which helps slow the movement of electrical signals through the AV node. This can reduce heart rate in horses with supraventricular arrhythmias such as atrial fibrillation. [1][3]
Uses in Horses
Digoxin is used off-label in horses for the management of congestive heart failure and selected cardiac arrhythmias. [2][3][4] Because its use in horses is extra-label, dosing recommendations and clinical applications are based on veterinary experience, published pharmacokinetic data, and data from other species. [2][5][6]
Congestive Heart Failure
The primary indication for digoxin in horses is congestive heart failure (CHF). [2][3] Congestive heart failure develops when the heart can no longer circulate blood effectively enough to meet the body’s needs. [10]
This may occur because the ventricles cannot contract efficiently, do not fill properly between beats, or because severe arrhythmias interfere with coordinated cardiac function. [10]
Conditions associated with CHF in horses include: [8][9][10][11]
- Valvular disease
- Myocardial disease
- Advanced cardiac dysfunction
As heart disease progresses, affected horses may develop signs such as: [14]
- Exercise intolerance
- Fatigue
- Abnormal respiratory effort
- Ventral edema (abdominal fluid retention)
- Coughing
- Collapse
- Reduced athletic performance
By increasing the force of heart muscle contractions, digoxin may help improve circulatory performance and reduce some clinical signs associated with poor cardiac output. [1][3]
Atrial Fibrillation
Digoxin is also used in horses with atrial fibrillation, particularly when control of ventricular response rate is desired. [3][7]
Its effects on AV nodal conduction can help: [3]
- Slow ventricular rate
- Improve hemodynamic stability in some horses
Digoxin is usually used alongside other antiarrhythmic therapies rather than as the sole treatment for atrial fibrillation. [3]
Administration Routes
Digoxin is administered orally or intravenously. [2] Oral administration is commonly used for long-term management of congestive heart failure or chronic arrhythmias, while intravenous administration may be used in hospitalized horses requiring a more rapid onset of action. [2]
Because digoxin use in horses is extra-label and the drug has a narrow therapeutic index, dosing must be individualized and guided by clinical response, serum concentrations when available, and careful patient monitoring. [2][5][6]
Storage
Digoxin products should be stored at controlled room temperature and protected from excessive moisture and light. [2]
Safety Warnings & Precautions
Digoxin has a narrow margin of safety and requires careful clinical oversight. Small changes in absorption, elimination, hydration status, or electrolyte balance can significantly alter serum drug concentrations. [2][5][6]
Important safety concerns include: [1][2]
- Development of potentially life-threatening arrhythmias
- Increased toxicity risk in horses with kidney disease
- Increased sensitivity in the presence of hypokalemia or other electrolyte disturbances
Drug accumulation may occur if elimination is impaired. Due to this, renal function plays an important role in digoxin safety because the drug is primarily eliminated through the kidneys. [2][6] Horses with existing kidney disease may require especially careful assessment before and during treatment.
Monitoring
Horses receiving digoxin should be monitored carefully throughout treatment. [2][3]
Clinicians may monitor: [2][3]
- Heart rate and rhythm
- Electrocardiographic (ECG) changes
- Appetite and gastrointestinal function
- Renal (kidney) parameters
- Electrolyte concentrations
- Clinical signs of toxicity (poisoning)
Measurement of serum digoxin concentrations may also be used to help assess therapeutic exposure and reduce the risk of toxicity. [5][6]
Contraindications
Contraindications refer to circumstances where a drug should be avoided or used with caution.
Digoxin should be avoided or used cautiously in horses with conditions that increase the risk of toxicity or arrhythmia development. [2]
Contraindications and major precautions include: [2]
- Ventricular arrhythmias
- Advanced AV block
- Hypersensitivity to cardiac glycosides
- Significant electrolyte abnormalities, particularly hypokalemia
Use cautiously in horses with: [2][3]
- Renal (kidney) dysfunction
- Existing conduction disturbances
- Severe systemic illness
Electrolyte imbalances can significantly alter sensitivity to digoxin and increase the likelihood of toxic effects. [1][2]
Side Effects
Side effects associated with digoxin commonly involve the gastrointestinal tract, cardiovascular system, and central nervous system. [2][3]
Potential side effects include: [2]
Cardiac toxicity is one of the most clinically important adverse effects associated with digoxin therapy. Both supraventricular and ventricular arrhythmias can occur. [1][3]
Human medical literature also describes neurologic and visual disturbances associated with digoxin toxicity. While these effects are not well-characterized in horses, they illustrate the broad systemic effects that can occur with excessive drug exposure. [1]
Acute Toxicity (Overdose)
Clinical signs of acute toxicity may include: [2][3]
- Severe arrhythmias
- Weakness or collapse
- Severe lethargy
- Gastrointestinal disturbances
Seek immediate veterinary attention if digoxin toxicity is suspected. Treatment may involve discontinuation of the drug, correction of electrolyte abnormalities, supportive care, and management of arrhythmias. [2][3] In human medicine, digoxin-specific antibody fragments are used in severe toxicity cases. [1] Use of these products in horses is not well-established.
When to Call the Veterinarian
Because digoxin has a narrow therapeutic index, changes during treatment should be taken seriously. Prompt veterinary evaluation can help determine whether a horse is experiencing worsening heart disease, adverse effects, or drug toxicity.
Call your veterinarian if your horse develops: [2][3]
- Weakness, collapse, or marked lethargy
- New or worsening arrhythmias
- Loss of appetite, diarrhea, or colic signs during treatment
- Any suspected overdose or sudden decline in health
Your veterinarian may evaluate heart rhythm, electrolyte status, kidney function, and serum digoxin concentrations to help determine whether the medication should be adjusted or discontinued.
Drug Interactions
Numerous medications and physiologic disturbances can alter digoxin activity or increase the risk of toxicity. [1][2]
Potential interactions include: [1][2]
- Drugs that alter electrolyte balance
- Antiarrhythmic drugs that affect cardiac conduction
- Medications that alter digoxin absorption or elimination
Hypokalemia is particularly important because low potassium concentrations increase myocardial sensitivity to digoxin and can also cause arrhythmias. [1][2] Hydration also matters, as horses with dehydration may be at greater risk of altered drug metabolism and related complications.
Concurrent administration of multiple cardiovascular medications may increase the risk of conduction disturbances or arrhythmias, making careful monitoring essential in horses receiving combination therapy. [3]
Regulatory Status & Legal Considerations
There are currently no equine-approved formulations of digoxin available. Veterinarians may prescribe human or compounded formulations for use in horses under extra-label guidelines.
Extra-label drug use is common in veterinary medicine because many medications are not specifically approved for all animal species. While this practice is legally permitted, it means manufacturers do not guarantee the safety or efficacy of the product in horses and are not responsible for outcomes associated with its use.
Status in Competition
Digoxin is classified as a Controlled Medication under FEI regulations. It may be used therapeutically but must not be present above permitted levels or within the detection period at the time of competition. [15]
Work with your veterinarian and always check the current regulations for your sport before starting your horse on a new medication.
Frequently Asked Questions
Here are some frequently asked questions about digoxin for horses:
Digoxin is used in horses to help manage congestive heart failure and certain cardiac arrhythmias, especially atrial fibrillation with a rapid ventricular response. It works best in carefully selected cardiovascular cases where a veterinarian wants to improve contractility or help slow conduction through the AV node. In modern equine practice, digoxin is used less often than in the past because it has a narrow safety margin. It is still an important prescription option in some horses under close veterinary supervision.
Digoxin works in horses by increasing the strength of heart muscle contractions and by slowing electrical conduction through the atrioventricular node. It does this by blocking sodium-potassium ATPase pumps in heart muscle cells, which increases intracellular calcium availability. That extra calcium can help the heart contract more forcefully. Digoxin also increases vagal activity, which can help reduce heart rate in some supraventricular arrhythmias.
Digoxin can help horses with atrial fibrillation by controlling ventricular response rate rather than directly curing the arrhythmia in every case. Its main role is to slow conduction through the AV node, which may improve hemodynamic stability in some horses. In practice, digoxin is often used alongside other antiarrhythmic therapies instead of as a stand-alone treatment. Whether it fits a particular atrial fibrillation case depends on the horse's rhythm, clinical status, and overall treatment plan.
Congestive heart failure in horses is a condition where the heart can no longer circulate blood effectively enough to meet the body's needs. Horses with congestive heart failure may develop exercise intolerance, fatigue, edema, abnormal respiratory effort, collapse, or poor performance. The condition can be linked to valvular disease, myocardial disease, or advanced cardiac dysfunction. Digoxin may be used in some of these cases to improve circulatory performance by increasing contractile force.
Digoxin is not approved in horse-specific formulations, so its use in horses is extra-label. That means veterinarians may prescribe human-grade or compounded formulations when they believe it is clinically appropriate. Extra-label use is common in veterinary medicine, but it also means dosing and monitoring require added care. Owners should only use digoxin exactly as directed by a licensed veterinarian.
Digoxin is given to horses either orally or intravenously, depending on the clinical situation. Oral dosing is more commonly used for longer-term management of congestive heart failure or chronic arrhythmias. Intravenous administration may be used in hospitalized horses when a faster onset is needed. The route, formulation, and schedule should always be determined by the attending veterinarian.
Digoxin requires careful monitoring in horses because the difference between a helpful dose and a toxic dose can be small. Kidney function, hydration status, electrolyte balance, and individual differences in absorption or elimination can all change how the drug behaves. For that reason, veterinarians may monitor heart rhythm, appetite, gastrointestinal signs, renal values, electrolytes, and sometimes serum digoxin concentrations. Close follow-up helps reduce the risk of dangerous complications.
The side effects of digoxin in horses commonly involve the gastrointestinal tract, cardiovascular system, and general attitude or energy level. Reported adverse effects include decreased appetite, diarrhea, colic signs, lethargy, depression, and cardiac arrhythmias. Cardiac toxicity is one of the most important concerns because digoxin itself can trigger dangerous rhythm disturbances. Any new signs that appear after starting treatment should be reported to a veterinarian promptly.
Signs of digoxin toxicity in horses can include severe arrhythmias, weakness, collapse, depression, and gastrointestinal upset. Toxicity may result from excessive dosing, impaired elimination, electrolyte disturbances, or drug interactions. Because the consequences can be life-threatening, suspected toxicity should be treated as a medical emergency. A veterinarian may stop the medication, correct electrolyte problems, provide supportive care, and manage any arrhythmias that develop.
You should call the veterinarian right away if your horse is taking digoxin and develops weakness, collapse, marked depression, new arrhythmias, loss of appetite, diarrhea, or colic signs. These changes may signal toxicity, worsening heart disease, or another serious treatment complication. Prompt evaluation is especially important because digoxin has a narrow therapeutic index. Your veterinarian may assess heart rhythm, kidney function, electrolytes, and drug exposure to decide whether treatment should be changed.
Horses may not be good candidates for digoxin if they have ventricular arrhythmias, advanced AV block, hypersensitivity to cardiac glycosides, or significant electrolyte abnormalities such as hypokalemia. Extra caution is also needed in horses with renal dysfunction, existing conduction disturbances, or severe systemic illness. These factors can increase the risk of toxicity or worsen arrhythmia problems. A full veterinary assessment is important before starting the drug.
Digoxin can interact with other medications or supplements that affect electrolyte balance, cardiac conduction, or the drug's absorption and elimination. Low potassium is especially important because hypokalemia can make the heart more sensitive to digoxin. Horses receiving multiple cardiovascular medications may need more intensive monitoring to reduce the risk of conduction disturbances or toxic effects. Owners should tell their veterinarian about all medications and supplements the horse is receiving before starting digoxin.
Horses taking digoxin may face competition restrictions because digoxin is classified as a Controlled Medication under FEI regulations. Therapeutic use may still be possible, but the drug must not be present above permitted levels or within the detection period at the time of competition. Rules can vary by discipline and governing body, so owners should confirm the current regulations before competing. A veterinarian can help plan treatment around medication-control requirements.
Summary
Digoxin is a cardiac glycoside used in horses to help manage congestive heart failure and certain cardiac arrhythmias, particularly atrial fibrillation.
- The drug increases the strength of heart contractions and can slow electrical conduction through the atrioventricular (AV) node
- Digoxin is used extra-label in horses because there are currently no equine-approved formulations available
- Horses with atrial fibrillation may develop poor performance, exercise intolerance, weakness, or abnormal heart rhythms
- Potential side effects include decreased appetite, diarrhea, lethargy, colic signs, and cardiac arrhythmias
- Kidney function, hydration status, electrolyte balance, and drug interactions can all influence the risk of digoxin toxicity
- Horses receiving digoxin may require monitoring of heart rhythm, clinical signs, electrolytes, and serum drug concentrations
References
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- Sleeper. M. M. Equine Cardiovascular Therapeutics. Veterinary Clinics of North America: Equine Practice. 2017. View Summary
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- Brumbaugh. G. W. et al. A Pharmacokinetic Study of Digoxin in the Horse. Journal of Veterinary Pharmacology and Therapeutics. 1983. View Summary
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- Reed. S. M. et al. Equine Internal Medicine 3rd ed. Saunders Elsevier. 2010.
- Cunningham. S. M. Heart Disease and Heart Failure in Horses. Merck Veterinary Manual. 2024.
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