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Rythmol SR Side Effects

Generic Name: propafenone

Note: This document contains side effect information about propafenone. Some of the dosage forms listed on this page may not apply to the brand name Rythmol SR.

In Summary

Common side effects of Rythmol SR include: constipation, dyspnea, nausea, and dysgeusia. Other side effects include: cardiac rate disturbance, death, atrial flutter, blurred vision, heart murmur, myasthenia, asthenia, diarrhea, first degree atrioventricular block, vomiting, and ecchymoses. See below for a comprehensive list of adverse effects.

For the Consumer

Applies to propafenone: oral capsule extended release, oral tablet

Along with its needed effects, propafenone (the active ingredient contained in Rythmol SR) may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking propafenone:

Less common Rare

Some side effects of propafenone may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

More common
  • Change in taste or bitter or metallic taste
Less common

For Healthcare Professionals

Applies to propafenone: oral capsule extended release, oral tablet


The Cardiac Arrhythmia Suppression Trial (CAST) revealed significantly higher mortality associated with some class IC antiarrhythmic agents in patients with a recent history (more than six days but less than two years prior to study) of myocardial infarction and asymptomatic non-life-threatening ventricular arrhythmias relative to placebo (5.1% versus 2.3%). Use of propafenone (the active ingredient contained in Rythmol SR) in this context is potentially harmful.

The safety and tolerability of propafenone after long-term administration has been reported. After 14 months, 60% of patients discontinued therapy, but only 15% discontinued therapy due to side effects. Overall, 59% of patients experienced at least 1 side effect, and the incidence was related to dose and age > 65 years. The overall incidence of side effects was not related to structural heart disease. However, cardiovascular toxicity including arrhythmia aggravation, heart failure, and serious conduction disturbances occurred more often in those with heart disease (20% vs. 13%).[Ref]


Accelerated ventricular tachycardias refractory to overdrive pacing in patients on propafenone (the active ingredient contained in Rythmol SR) who had arrhythmias previously responsive to overdrive pacing, have been reported.[Ref]

Cardiovascular side effects may be serious. Two to four percent of patients have experienced a proarrhythmic effect. Propafenone slows conduction throughout the myocardial conduction system with very little effect on repolarization, resulting in a prolonged PR interval and QRS complex. Cases of bradycardia, sinus pauses and arrest, accelerated reentrant tachycardia, AV block, and ventricular tachycardia, including torsades de pointes, have been reported.

Propafenone has negative inotropic activity. Preexisting congestive heart failure has been aggravated in 9% of patients and new congestive heart failure induced in 5% of patients.[Ref]

Nervous system

A 68 year-old man with ocular myasthenia gravis developed generalized myasthenia with ptosis, diplopia, dysarthria, dysphagia, and limb weakness within hours after beginning propafenone (the active ingredient contained in Rythmol SR) 450 mg per day. It is believed that propafenone, by blocking fast sodium channels, may interfere with the generation of propagation of the motor end-plate potential.[Ref]

Nervous system side effects have included dizziness in 7%, headaches in 5%, ataxia in 1% to 3%, and fatigue in less than 1% of patients. Rare cases of exacerbation of myasthenia gravis and peripheral neuropathy have been associated with the use of propafenone.[Ref]


Gastrointestinal side effects have included general gastrointestinal upset in 3%, constipation in 4%, a metallic taste in 9%, and nausea or vomiting in approximately 2% of patients.[Ref]


Respiratory side effects have included dyspnea, wheezing, and bronchoconstriction. These side effects have not exclusively been reported in patients with preexisting reactive airways disease.[Ref]

Limited data indicate that the plasma concentration of propafenone is not correlated with respiratory complaints, although caution is recommended particularly when daily doses exceed 450 mg. Propafenone is associated with a significant decrease in the average dose of methacholine required to reduce the forced expiratory volume in 1 second (FEV1) by 20% and an increase in the use of beta-agonist inhalers in patients with asthma.

A case of wheezing and decreased expiratory flow rates has been associated with propafenone in a 50-year-old woman with no history of reactive airways disease and who had previously received atenolol and metoprolol without problems.[Ref]


Profound neutropenia associated with bone marrow evidence of myeloid injury has been reported in at least 4 cases. Each patient recovered completely within 7 to 30 days after drug withdrawal.[Ref]

Hematologic side effects have been rare. A meta-analysis of all adverse drug events associated with propafenone yielded 4 cases of agranulocytosis. The reported rate is 1 case per 10,000 prescriptions per year. Anemia, granulocytopenia, increased bleeding time, leukopenia, purpura, and thrombocytopenia have occurred.[Ref]


Propafenone associated liver injury appears to be secondary to hepatocellular injury, cholestasis, or a combination of these. In some cases an allergic reaction has been suspected, while in at least one case, an idiosyncratic toxicity of propafenone (the active ingredient contained in Rythmol SR) metabolites in the biliary epithelial cells was suspected. There are no known fatalities or cases in which the drug had to be discontinued due to elevated liver function tests.[Ref]

Hepatic side effects been reported. The overall incidence of hepatotoxicity is estimated to be 0.1% to 0.2%.[Ref]


A 63-year-old woman with hypertension, coronary artery disease, and ventricular tachycardia developed a facial photosensitive rash, generalized erythema, and an elevated ANA titer in a homogenous and speckled pattern within 2 months after beginning propafenone (the active ingredient contained in Rythmol SR) 300 mg every 8 hours. The clinical and laboratory abnormalities resolved within 1 month of drug discontinuation and reappeared upon rechallenge with propafenone.[Ref]

Immunologic side effects including the rare development of a lupus-like syndrome have been reported in at least two cases. An elevated ANA titer has been reported in 0.7% of patients.[Ref]


A 39-year-old woman with a history of congestive heart failure and symptomatic premature ventricular depolarizations developed paresthesias, insomnia, paranoia, hallucinations, and frank psychosis within 24 hours after starting propafenone (the active ingredient contained in Rythmol SR) 300 mg every 12 hours. The syndrome resolved within 2 to 3 days after discontinuation of the drug and institution of haloperidol. The patient subsequently did well off of haloperidol, on an alternative antiarrhythmic agent.

A 61-year-old man with a history of sick sinus syndrome, chronic atrial fibrillation, and premature ventricular depolarizations developed amnesia and disorientation within six days after starting propafenone. His mental status deterioration resolved within six to seven hours after discontinuing therapy. Comparable adverse effects have been associated with an analogous agent, propranolol.[Ref]

Psychiatric abnormalities have been limited to a case of frank psychosis and a case of global amnesia.[Ref]


Ocular side effects of blurred vision (4%), abnormal vision(2%), and eye irritation (less than 1%) have been reported.[Ref]


Dermatologic side effects including at least one case of acute generalized exanthematous pustulosis, which resolved within 3 days after discontinuation of propafenone (the active ingredient contained in Rythmol SR) has been reported.[Ref]


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Some side effects of Rythmol SR may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.