Tambocor Retard: Uses, Dosage & Side Effects
Extended-release antiarrhythmic for heart rhythm disorders (flecainide)
Quick Facts About Tambocor Retard
Key Takeaways About Tambocor Retard
- For supraventricular arrhythmias: Tambocor Retard is primarily used for paroxysmal atrial fibrillation, atrial flutter, and paroxysmal supraventricular tachycardia in patients without significant structural heart disease
- Not for post-MI patients: The CAST trial demonstrated increased mortality with flecainide in patients after myocardial infarction — it must not be used in patients with heart attack history or heart failure
- Once-daily dosing: The extended-release formulation allows convenient once-daily administration at approximately the same time each day, improving treatment adherence
- ECG monitoring essential: Regular ECG monitoring is required to assess QRS widening and PR prolongation, particularly at treatment initiation and dose changes
- Electrolyte correction first: Potassium and other electrolyte imbalances must be corrected before starting treatment, as they increase the risk of pro-arrhythmic effects
What Is Tambocor Retard and What Is It Used For?
Tambocor Retard is an extended-release antiarrhythmic medication containing flecainide acetate. It slows electrical conduction in the heart to treat and prevent abnormal heart rhythms (arrhythmias), particularly those originating in the upper chambers of the heart (supraventricular arrhythmias).
Flecainide belongs to the class IC antiarrhythmic drugs according to the Vaughan-Williams classification. It was developed in the 1970s and has been used clinically since the early 1980s to manage various cardiac rhythm disturbances. The drug is included in the European Society of Cardiology (ESC) guidelines as a first-line pharmacological option for rhythm control in atrial fibrillation in patients without significant structural heart disease.
Tambocor Retard is the modified-release (extended-release) formulation of flecainide, designed to provide sustained plasma drug levels over 24 hours. This allows for once-daily dosing, which offers a significant advantage over the standard immediate-release tablet formulation (Tambocor), which typically requires twice-daily administration. The retard capsule contains microspheres that slowly release flecainide in the gastrointestinal tract, ensuring a steady therapeutic concentration.
The mechanism of action of flecainide involves blocking fast sodium channels in the cardiac cell membrane. By doing so, it slows the rate of depolarization (phase 0) of the cardiac action potential, reducing conduction velocity throughout the heart. It is particularly effective at slowing conduction in the atria, the atrioventricular (AV) node, the His-Purkinje system, and in accessory pathways (such as those found in Wolff-Parkinson-White syndrome). This slowed conduction helps to interrupt and prevent re-entrant circuits that underlie many arrhythmias.
Tambocor Retard is primarily prescribed for the following conditions:
- Paroxysmal atrial fibrillation (PAF) — intermittent episodes of irregular, rapid heart rhythm originating in the atria. Flecainide can be used both for ongoing prophylaxis and as a “pill-in-the-pocket” strategy for terminating acute episodes
- Paroxysmal atrial flutter — a related condition with a rapid but more organized atrial rhythm
- Paroxysmal supraventricular tachycardia (PSVT) — episodes of abnormally fast heart rate arising from above the ventricles, including AV nodal re-entrant tachycardia (AVNRT) and AV re-entrant tachycardia (AVRT)
- Arrhythmias associated with accessory pathways — including Wolff-Parkinson-White (WPW) syndrome
Flecainide should generally be used only in patients without significant structural heart disease. The landmark Cardiac Arrhythmia Suppression Trial (CAST), published in 1991, demonstrated that flecainide increased mortality when used to suppress premature ventricular contractions in patients who had recently suffered a myocardial infarction. As a result, flecainide is contraindicated in patients with a history of heart attack, heart failure, or significant left ventricular dysfunction.
Pharmacokinetics of the extended-release formulation
Flecainide is well absorbed after oral administration, with a bioavailability of approximately 95%. The extended-release capsule formulation provides a smoother pharmacokinetic profile compared to the immediate-release tablet. Peak plasma concentrations are reached later and the trough-to-peak ratio is more favorable, resulting in fewer fluctuations in drug levels throughout the day.
Flecainide is metabolized primarily in the liver by the cytochrome P450 enzyme CYP2D6. Approximately 7–10% of the Caucasian population are “poor metabolizers” of CYP2D6 substrates, meaning they break down flecainide more slowly and may have higher plasma levels at standard doses. About 30% of the drug is excreted unchanged by the kidneys. The elimination half-life is 12–27 hours, which supports once-daily dosing with the extended-release formulation.
What Should You Know Before Taking Tambocor Retard?
Tambocor Retard must not be used in patients with heart failure, history of myocardial infarction, conduction disorders (without pacemaker), haemodynamically significant valvular disease, or Brugada syndrome. Thorough cardiac evaluation is required before starting treatment.
Before prescribing Tambocor Retard, your physician will perform a comprehensive cardiac assessment. This typically includes a 12-lead ECG, echocardiography to assess cardiac structure and function, and blood tests to check electrolyte levels and kidney and liver function. These investigations are essential to identify patients who may be at increased risk of adverse effects, particularly pro-arrhythmic events.
Contraindications
Tambocor Retard must not be used in the following situations:
- Hypersensitivity to flecainide or any of the excipients in the formulation
- Heart failure (current or history of) — flecainide has negative inotropic effects that can worsen cardiac function
- History of myocardial infarction (heart attack) — the CAST trial demonstrated increased mortality in this population
- Cardiac conduction disorders — including second-degree or third-degree AV block, right bundle branch block combined with left hemiblock (bifascicular block), and sinus node dysfunction, unless a pacemaker is fitted
- Haemodynamically significant valvular heart disease — structural valve abnormalities can alter drug distribution and increase risk
- Brugada syndrome — a genetic channelopathy affecting the heart’s sodium channels, which can be unmasked or worsened by flecainide
- Beta-blocker therapy for heart failure — the combination with flecainide is contraindicated in heart failure patients
- Permanent (chronic) atrial fibrillation in certain cases — where the risks of cardioversion outweigh the benefits
Warnings and Precautions
Speak to your doctor before taking Tambocor Retard if you have any of the following:
- Any other heart condition — even mild structural abnormalities may increase the risk of pro-arrhythmic effects
- Abnormally slow heart rate (bradycardia) or very low blood pressure — flecainide may further suppress heart rate and cardiac output
- Pacemaker — flecainide can affect pacing thresholds, and pacemaker parameters may need to be reprogrammed after starting treatment
- Impaired liver function — since flecainide is metabolized in the liver, reduced hepatic function can lead to drug accumulation. Plasma level monitoring is recommended
- Impaired kidney function — significant renal impairment slows the elimination of unchanged flecainide. Dose reduction and plasma level monitoring are recommended
Like all antiarrhythmic drugs, flecainide has the potential to cause new arrhythmias or worsen existing ones (pro-arrhythmic effect). This risk is highest in patients with structural heart disease, electrolyte imbalances (particularly low potassium or magnesium), or when plasma drug levels exceed the therapeutic range. For certain types of arrhythmias, treatment must be initiated by a specialist, and hospital admission may be necessary for monitoring during the initial dosing period.
- Very fast or irregular heartbeat, palpitations — may indicate a pro-arrhythmic effect
- Chest pain or severe breathlessness — could indicate worsening heart function
- Fainting or severe dizziness — may signal dangerously low heart rate or blood pressure
- Widespread rash or signs of allergic reaction — discontinue and seek medical attention immediately
Pregnancy and Breastfeeding
Flecainide crosses the placenta and may potentially affect the developing fetus. Tambocor Retard should only be used during pregnancy if the potential benefit clearly outweighs the risk to the unborn child. In specialist settings, flecainide has been used to treat fetal supraventricular tachycardia (an abnormally fast heart rate in the unborn baby), but this requires close fetal monitoring by a maternal-fetal medicine specialist.
Flecainide is excreted in breast milk, although the amount reaching the infant is generally considered unlikely to cause significant effects. However, breastfeeding should only be undertaken with medical approval while taking Tambocor Retard, and the infant should be monitored for any signs of cardiac effects.
Children and Adolescents
Tambocor Retard (the extended-release formulation) is not recommended for children under 12 years of age due to insufficient clinical experience in this age group. The standard immediate-release formulation of flecainide is used in pediatric cardiology under specialist supervision for certain supraventricular arrhythmias, with careful weight-based dosing and monitoring.
Driving and Operating Machinery
Some patients may experience side effects such as dizziness or visual disturbances (blurred vision, difficulty focusing) while taking Tambocor Retard. These effects can impair the ability to drive or operate machinery safely. Patients should assess their individual response to the medication before engaging in activities requiring alertness and clear vision. If affected, they should refrain from driving until the symptoms resolve.
How Does Tambocor Retard Interact with Other Drugs?
Tambocor Retard interacts with several medications, particularly other cardiac drugs, certain antidepressants, and drugs that affect the CYP2D6 enzyme system. Some combinations are contraindicated, while others require dose adjustments or increased monitoring.
Drug interactions with flecainide are clinically significant because they can alter plasma drug levels (leading to either reduced efficacy or increased toxicity) or produce additive cardiac effects (such as further slowing of conduction or heart rate suppression). The principal metabolic pathway involves CYP2D6, and inhibitors of this enzyme can substantially increase flecainide plasma concentrations.
It is essential to inform your doctor about all medications you are currently taking, including over-the-counter medicines, herbal remedies, and supplements, before starting Tambocor Retard.
Major Interactions
| Drug / Class | Risk | Recommendation |
|---|---|---|
| Amiodarone | Increases flecainide plasma levels by ~33%; additive cardiac conduction effects | Reduce flecainide dose by 50% if combination necessary; monitor ECG closely |
| Quinidine | Inhibits CYP2D6, increasing flecainide levels; additive QRS prolongation | Avoid combination; increased risk of pro-arrhythmia |
| Verapamil | Additive negative inotropic and conduction-slowing effects | Do not combine in patients with impaired cardiac function |
| Beta-blockers (in heart failure) | Additive negative inotropic effects; risk of severe bradycardia | Contraindicated in heart failure; monitor closely in other patients |
| Mizolastine, Terfenadine | Risk of QT prolongation and cardiac arrhythmias | Avoid combination |
| Ritonavir (antiviral) | Potent CYP enzyme inhibitor, may markedly increase flecainide levels | Avoid combination; risk of serious toxicity |
Other Important Interactions
| Drug / Class | Effect | Action Required |
|---|---|---|
| Digoxin | Flecainide increases digoxin plasma levels by 15–25% | Monitor digoxin levels; reduce digoxin dose if needed |
| Fluoxetine, Paroxetine (SSRIs) | CYP2D6 inhibition increases flecainide levels | Monitor for flecainide toxicity; consider dose reduction |
| Phenytoin, Carbamazepine, Phenobarbital | CYP enzyme induction decreases flecainide levels | Monitor efficacy; may need flecainide dose increase |
| Cimetidine | Inhibits CYP enzymes; increases flecainide levels by ~30% | Monitor; consider alternative H2 antagonist (e.g. ranitidine) |
| Bupropion | CYP2D6 inhibition increases flecainide plasma levels | Monitor for toxicity; dose adjustment may be needed |
| Terbinafine | CYP2D6 inhibition increases flecainide levels | Monitor ECG and plasma levels during concurrent use |
| Diuretics, Corticosteroids, Laxatives | May cause hypokalaemia, increasing pro-arrhythmic risk | Monitor and correct electrolyte levels |
| Quinine (antimalarial) | May increase flecainide plasma levels | Monitor; avoid combination if possible |
| Clozapine | Additive cardiac effects; risk of arrhythmias | Use with caution; ECG monitoring recommended |
| Tricyclic antidepressants | Additive cardiac conduction effects; CYP2D6 interaction | Monitor ECG; consider dose adjustments |
What Is the Correct Dosage of Tambocor Retard?
The usual adult dose of Tambocor Retard is 200 mg once daily. The capsule should be swallowed whole at approximately the same time each day. Dose adjustments may be needed for elderly patients and those with kidney or liver impairment.
Tambocor Retard should always be taken exactly as prescribed by your doctor. The extended-release capsules must be swallowed whole and not opened, crushed, or chewed, as this would destroy the modified-release mechanism and lead to an immediate release of the full dose, potentially causing toxicity. Treatment is typically initiated by a cardiologist or a physician experienced in managing cardiac arrhythmias.
Adults
Standard Adult Dosage
200 mg once daily (one 200 mg capsule, or two 100 mg capsules), taken at approximately the same time each day. The dose is individualized based on clinical response and ECG monitoring. Some patients may be adequately controlled on 100 mg once daily, while the maximum recommended dose should not exceed 400 mg daily (given as two divided doses of the immediate-release formulation if needed).
Before starting treatment and after each dose adjustment, your doctor will perform an ECG to monitor the QRS duration and PR interval. If the QRS duration increases by more than 25% from baseline, or exceeds 180 ms, the dose should be reduced or the drug discontinued. Plasma drug level monitoring may be performed, with the target therapeutic range for flecainide being 200–1,000 ng/mL.
Children and Adolescents
Pediatric Dosage
Tambocor Retard (extended-release capsules) is not recommended for children under 12 years of age due to lack of clinical data. The standard immediate-release formulation may be used in children under specialist supervision, with dosing typically based on body weight (1–3 mg/kg/day in divided doses, up to a maximum of 8 mg/kg/day for infants).
Elderly Patients and Renal/Hepatic Impairment
Dose Adjustments
Elderly patients and patients with significant renal impairment may require a lower starting dose, as clearance of flecainide is reduced. In severe renal impairment (creatinine clearance < 35 mL/min), the maximum recommended dose is 100 mg daily, and plasma level monitoring is strongly advised. For patients with hepatic impairment, dose reduction and plasma level monitoring are also recommended, as metabolism may be significantly impaired.
Missed Dose
If you forget to take a dose of Tambocor Retard, take it as soon as you remember — unless it is nearly time for your next scheduled dose. In that case, skip the missed dose and take the next one at the usual time. Do not take a double dose to make up for a forgotten one, as this significantly increases the risk of serious cardiac side effects.
Overdose
Flecainide overdose is a medical emergency that can be life-threatening. Symptoms may include severe cardiac conduction abnormalities, bradycardia, hypotension, heart failure, cardiac arrest, and respiratory failure. There is no specific antidote for flecainide overdose. Treatment is supportive and may include intravenous sodium bicarbonate (which can partially reverse the sodium channel blockade), cardiac pacing for severe bradycardia, inotropic support, and in extreme cases, extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass.
If you suspect an overdose, contact emergency services immediately or go to the nearest hospital emergency department. Do not wait for symptoms to develop, as cardiac toxicity can progress rapidly.
What Are the Side Effects of Tambocor Retard?
The most common side effects of Tambocor Retard are dizziness, lightheadedness, and visual disturbances (blurred vision, difficulty focusing). These are usually transient and may improve with continued treatment. Serious side effects include pro-arrhythmic effects, heart failure, and cardiac arrest.
Like all medicines, Tambocor Retard can cause side effects, although not everybody experiences them. The side effects are listed below by frequency, based on clinical trial data and post-marketing surveillance. It is important to report any new or worsening symptoms to your doctor promptly, particularly any changes in heart rhythm, breathing difficulties, or visual disturbances.
Very Common
May affect more than 1 in 10 people
- Dizziness, lightheadedness, feeling unsteady
- Visual disturbances — blurred vision, difficulty focusing (these are usually temporary and may resolve with continued treatment)
Common
May affect up to 1 in 10 people
- General weakness (asthenia), extreme tiredness (fatigue)
- Fever
- Fluid retention (oedema)
- New or worsened heart rhythm disturbances (pro-arrhythmic effect)
- Shortness of breath (dyspnoea)
Uncommon
May affect up to 1 in 100 people
- Reduction in red blood cells, white blood cells, and platelets
- Nausea, vomiting, constipation, abdominal pain
- Loss of appetite, diarrhoea, indigestion, flatulence
- Atrial flutter with 1:1 conduction (in patients with atrial flutter)
- Allergic skin reactions (rash) and hair loss
Rare
May affect up to 1 in 1,000 people
- Elevated liver enzymes, occasionally with jaundice (reversible on stopping treatment)
- Depression, anxiety, insomnia
- Confusion, hallucinations, memory loss
- Paraesthesia (tingling/numbness), impaired coordination, movement difficulties
- Tremor, reduced sense of touch
- Increased sweating, flushing
- Drowsiness, fainting due to sudden blood pressure drop
- Headache, peripheral neuropathy, seizures
- Vertigo and tinnitus (ringing in the ears)
- Pneumonitis (lung inflammation)
- Severe urticaria (hives)
Very Rare
May affect up to 1 in 10,000 people
- Increase in certain antibodies (antinuclear antibodies)
- Corneal deposits in the eye
- Photosensitivity (skin reaction to sunlight)
Not Known
Frequency cannot be estimated from available data
- Various cardiac rhythm disturbances including heart block, cardiac arrest, heart failure, and myocardial infarction
- Chest pain
- Impaired liver function (hepatic dysfunction)
- Pulmonary fibrosis (scarring of the lungs, causing breathlessness and reduced oxygen uptake)
- Various dose-related arrhythmias
If you experience any side effects, including those not listed above, talk to your doctor or pharmacist. You can also report side effects directly to your national pharmacovigilance authority. By reporting side effects, you help provide more information on the safety of this medicine.
How Should You Store Tambocor Retard?
Store Tambocor Retard below 25°C in its original packaging, out of the reach and sight of children. Do not use after the expiry date printed on the packaging.
Proper storage of medication is essential to maintain its effectiveness and safety. Tambocor Retard should be kept at a temperature not exceeding 25°C (77°F). Store the capsules in their original blister packaging to protect them from moisture and light. Do not transfer them to other containers.
Keep this medicine out of the sight and reach of children. Children are particularly vulnerable to the cardiac effects of flecainide, and accidental ingestion could be life-threatening. If a child accidentally swallows this medication, seek emergency medical attention immediately.
Do not use Tambocor Retard after the expiry date stated on the blister pack and carton. The expiry date refers to the last day of that month. Do not dispose of medicines via household waste or wastewater. Return unused medicines to your pharmacy for proper disposal, which helps to protect the environment.
What Does Tambocor Retard Contain?
Tambocor Retard capsules contain flecainide acetate as the active ingredient, along with several inactive excipients that form the extended-release matrix and capsule shell.
Active Ingredient
Each Tambocor Retard capsule contains flecainide acetate as the active substance. The capsules are available in two strengths: 100 mg and 200 mg.
Inactive Ingredients (Excipients)
The inactive ingredients serve various roles in the formulation, including forming the extended-release matrix, providing the capsule shell, and ensuring stability:
- Microcrystalline cellulose — filler and matrix former
- Methacrylic acid-methyl methacrylate copolymer — controlled-release coating polymer
- Macrogol 400 — plasticizer for the coating
- Talc — anti-adherent and glidant
Capsule shell components:
- Gelatin
- Titanium dioxide (E 171) — white colouring
- Black iron oxide (E 172) — colouring (both strengths)
- Erythrosine (E 127) — pink colouring (200 mg strength only)
Printing ink: Shellac, black iron oxide (E 172), propylene glycol, ammonium hydroxide, potassium hydroxide.
Appearance
Tambocor Retard 100 mg capsules are grey and white, marked with “M” and “100”. Tambocor Retard 200 mg capsules are grey and pink, marked with “M” and “200”. The capsules are supplied in blister packs of 30 or 60 units.
Frequently Asked Questions About Tambocor Retard
Tambocor Retard is used to treat and prevent abnormal heart rhythms (arrhythmias), primarily supraventricular arrhythmias. These include paroxysmal atrial fibrillation, paroxysmal atrial flutter, and paroxysmal supraventricular tachycardia. It contains flecainide acetate, a class IC antiarrhythmic drug that works by slowing electrical conduction in the heart. The “Retard” formulation is an extended-release capsule designed for convenient once-daily dosing.
The key difference is the release mechanism. Tambocor Retard is a modified-release (extended-release) capsule that slowly releases flecainide over 24 hours, allowing once-daily dosing. Regular Tambocor is an immediate-release tablet that is usually taken twice daily. The Retard formulation provides more stable plasma drug levels with fewer peaks and troughs, which may improve tolerability. The capsules must be swallowed whole and never crushed or chewed.
The most serious side effect is the potential to cause new or worsened cardiac arrhythmias (pro-arrhythmic effect), which can be life-threatening. This risk is particularly elevated in patients with structural heart disease, which is why Tambocor Retard is contraindicated in patients with heart failure, history of myocardial infarction, or significant structural heart abnormalities. Other serious side effects include cardiac arrest, severe bradycardia, and heart failure. Common but less serious side effects include dizziness and visual disturbances.
Tambocor Retard must not be used by patients with heart failure (current or history), history of myocardial infarction (heart attack), significant cardiac conduction disorders (unless a pacemaker is fitted), haemodynamically significant valvular heart disease, or Brugada syndrome. It is also contraindicated in patients taking beta-blockers for heart failure. The landmark CAST trial showed that flecainide increased mortality in post-myocardial infarction patients, which is why it is restricted to patients without significant structural heart disease.
Tambocor Retard should only be used during pregnancy if the potential benefit justifies the risk to the fetus. Flecainide crosses the placenta and could potentially affect the developing baby. In specialist settings, flecainide has been used to treat fetal supraventricular tachycardia, but this requires expert monitoring. During breastfeeding, flecainide is excreted in breast milk, though the amount is generally considered unlikely to harm the infant. Nevertheless, breastfeeding while taking Tambocor Retard should only occur with medical approval.
Regular ECG monitoring is essential, particularly at treatment initiation and after dose adjustments. The QRS duration and PR interval should be assessed — if QRS widens by more than 25% from baseline, dose reduction or discontinuation may be necessary. Electrolyte levels (especially potassium) should be checked before starting and periodically during treatment. Liver and kidney function tests are recommended, especially for patients with pre-existing impairment. Plasma flecainide levels may be monitored in patients with severe renal or hepatic impairment, with a target therapeutic range of 200–1,000 ng/mL.
References & Medical Sources
All information on this page is based on peer-reviewed medical literature, international clinical guidelines, and official drug prescribing information. No pharmaceutical company sponsorship or commercial funding has influenced this content.
- The Cardiac Arrhythmia Suppression Trial (CAST) Investigators. “Preliminary report: effect of encainide and flecainide on mortality in a randomized trial of arrhythmia suppression after myocardial infarction.” New England Journal of Medicine, 321(6): 406–412, 1989. DOI: 10.1056/NEJM198908103210629
- Hindricks G, et al. “2020 ESC Guidelines for the diagnosis and management of atrial fibrillation.” European Heart Journal, 42(5): 373–498, 2021. DOI: 10.1093/eurheartj/ehaa612
- Brugada J, et al. “2019 ESC Guidelines for the management of patients with supraventricular tachycardia.” European Heart Journal, 41(5): 655–720, 2020. DOI: 10.1093/eurheartj/ehz467
- January CT, et al. “2019 AHA/ACC/HRS Focused Update of the 2014 AHA/ACC/HRS Guideline for the Management of Patients With Atrial Fibrillation.” Journal of the American College of Cardiology, 74(1): 104–132, 2019. DOI: 10.1016/j.jacc.2019.01.011
- British National Formulary (BNF). “Flecainide acetate.” National Institute for Health and Care Excellence (NICE). Updated 2025.
- European Medicines Agency (EMA). Flecainide acetate — Summary of Product Characteristics (SmPC). Accessed January 2026.
- World Health Organization (WHO). Model List of Essential Medicines. 23rd list, 2023.
- Tamargo J, et al. “Pharmacology of cardiac potassium channels.” Cardiovascular Research, 62(1): 9–33, 2004. DOI: 10.1016/j.cardiores.2003.12.026
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