Flecainide Side Effects

Not all side effects for flecainide may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.

For the Consumer

Applies to flecainide: oral capsule extended release, oral tablet

In addition to its needed effects, some unwanted effects may be caused by flecainide. In the event that any of these side effects do occur, they may require medical attention.

You should check with your doctor immediately if any of these side effects occur when taking flecainide:

More common
  • Difficult or labored breathing
  • dizziness, fainting, or lightheadedness
  • fast, irregular, pounding, or racing heartbeat or pulse
  • shortness of breath
  • tightness in the chest
  • wheezing
Less common
  • Burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
  • chest pain
  • fainting
  • feeling of warmth
  • fever
  • increased sweating
  • partial or slight paralysis
  • redness of the face, neck, arms, and occasionally, upper chest
  • shakiness and unsteady walk
  • shakiness in the legs, arms, hands, or feet
  • swelling of the feet or lower legs
  • trembling or shaking of the hands or feet
  • unsteadiness, trembling, or other problems with muscle control or coordination
  • Arm, back, or jaw pain
  • black, tarry stools
  • bleeding gums
  • blood in the urine or stools
  • blurred vision
  • chest discomfort
  • chest tightness or heaviness
  • chills
  • confusion
  • convulsions
  • cough
  • decrease in the frequency of urination
  • decrease in urine volume
  • difficulty in passing urine (dribbling)
  • difficulty with breathing
  • dizziness, faintness, or lightheadedness when getting up suddenly from a lying or sitting position
  • frequent urination
  • general feeling of discomfort or illness
  • headache
  • increased volume of pale, dilute urine
  • nausea
  • nervousness
  • noisy breathing
  • painful or difficult urination
  • pinpoint red spots on the skin
  • pounding in the ears
  • sensation of pins and needles
  • slow or fast heartbeat
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • stabbing pain
  • sweating
  • swollen glands
  • thickening of bronchial secretions
  • troubled breathing
  • unusual bleeding or bruising
  • unusual tiredness or weakness
  • yellow eyes or skin

Some of the side effects that can occur with flecainide may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:

More common
  • Blurred vision or seeing spots
Less common
  • Abdominal or stomach pain
  • acid or sour stomach
  • anxiety or mental depression
  • belching
  • constipation
  • continuing ringing or buzzing or other unexplained noise in the ears
  • depression
  • diarrhea
  • feeling of constant movement of self or surroundings
  • general feeling of discomfort or illness
  • hearing loss
  • heartburn
  • indigestion
  • lack or loss of strength
  • loss of appetite
  • sensation of spinning
  • sleepiness or unusual drowsiness
  • sleeplessness
  • skin rash
  • stomach discomfort, upset, or pain
  • trouble with sleeping
  • unable to sleep
  • weight loss
  • Bloated
  • change in color vision
  • change in taste
  • cracks in the skin
  • decreased awareness or responsiveness
  • decreased interest in sexual intercourse
  • difficulty seeing at night
  • difficulty with moving
  • dry mouth
  • excess air or gas in the stomach or intestines
  • eye pain or irritation
  • false or unusual sense of well-being
  • feeling of unreality
  • full feeling
  • hair loss or thinning of the hair
  • hives or welts
  • inability to have or keep an erection
  • increased sensitivity of the eyes to sunlight
  • itching skin
  • joint pain
  • lack of feeling or emotion
  • loss in sexual ability, desire, drive, or performance
  • loss of heat from the body
  • loss of memory
  • muscle aching or cramping
  • muscle pain or stiffness
  • passing gas
  • problems with memory
  • red, swollen skin
  • scaly skin
  • sense of detachment from self or body
  • severe sleepiness
  • swollen joints
  • swollen lips, mouth, or tongue
  • uncaring uncontrolled eye movements

For Healthcare Professionals

Applies to flecainide: oral tablet


The Cardiac Arrhythmia Suppression Trial (CAST) revealed significantly higher mortality associated with flecainide in patients with a recent history (more than six days but less than two years prior to study) of myocardial infarction (MI) and non-life-threatening ventricular ectopy relative to placebo (5.1% versus 2.3%). The risk of death relative to placebo in patients with a recent history of Q-wave MI and non-Q-wave MI is 8.7 and 1.7, respectively. Use of flecainide in this context is potentially harmful.

Side effects are more likely when plasma flecainide concentrations are greater than 1.0 mcg/mL.


Risk factors for a proarrhythmic effect include underlying congenital or structural heart disease.

A case of "pseudoinfarction" has been reported in which flecainide induced a transient right bundle branch block with a focal block in the septal fibers of the left bundle branch system. An electrocardiogram (ECG) also revealed ST segment elevations and a Q-wave pattern, consistent with septal infarction. The patient did not have a myocardial infarction by enzyme studies, and the ECG abnormalities resolved after discontinuation of flecainide.

One patient with a history of ischemic congestive heart failure, myocardial infarction (MI), and ventricular arrhythmias developed profound cardiogenic shock without evidence of MI or a new or worsened ventricular arrhythmia. The associated serum flecainide concentration was 1.8 mcg/mL.

Cardiovascular side effects including arrhythmias are the most serious side effects. Flecainide may cause or exacerbate arrhythmias in 1% of patients with preexisting paroxysmal supraventricular tachycardia and in 7% of patients with paroxysmal atrial fibrillation. Flecainide may also exacerbate arrhythmias in 7% to 13% of patients with preexisting sustained or nonsustained ventricular arrhythmias.

Flecainide-induced arrhythmias include sinus bradycardia or arrest in 2%, bundle branch blocks in 1%, increased premature ventricular depolarizations in 1%, ventricular tachycardia or fibrillation in 0.5%, and sudden death in 0.2% of patients. New ventricular arrhythmias have been reported in 3.4% of patients.

Flecainide may cause prolongation of the PR, QRS, and corrected QT intervals. Most of the QT interval prolongation is attributable to widening of the QRS complex rather than prolongation of the JT interval. Rare cases of torsades de pointes have been reported.

Exacerbation of congestive heart failure is rare and only occurs in about 0.5% and 9% of patients with preexisting supraventricular arrhythmias and ventricular arrhythmias, respectively. Hypotension is almost exclusively associated with intravenous administration of flecainide.

Nervous system

Flecainide may exacerbate myasthenia gravis.

At least 6 cases of flecainide-induced peripheral neuropathy (sensory loss) have been reported and it appears to develop after prolonged use ( 2 to 10 years). Following discontinuation of flecainide therapy, symptoms (e.g., lower-extremity weakness and/or paresthesias, gait disturbance) resolved over 3 to 6 months. However, in some cases the neuropathy did not resolve after discontinuation of flecainide.

Nervous system side effects, such as dizziness and visual disturbances (including blurred vision, decreased acuity, and scotomata) occur in 13% to 28% of patients who are taking flecainide doses of 400 mg per day. Transient headaches, asthenia, feelings of a thick tongue or lips, fatigue, paresthesias, and tremors have been reported in 2% to 10% of patients.


Gastrointestinal side effects include abdominal pain, nausea, and constipation in 1% to 4% of patients. Diarrhea occurs rarely.


Musculoskeletal side effects including weakness has been reported and may be more likely in patients with underlying muscular disorders.

A 33-year-old woman with atrial fibrillation, mitral valve prolapse, and a congenital muscle fiber disproportion myopathy developed muscle weakness which partially resolved after flecainide dosage reduction and completely resolved after substitution of flecainide with other antiarrhythmic agents.


High performance liquid chromatographic analysis of excised corneal deposits in one patient revealed opacities with the same chromatographic characteristics of flecainide.

Ocular side effects are limited to rare cases of corneal deposits.


Genitourinary side effects including complaints of impotence are reported in 4% of patients. A case of urinary retention associated with flecainide has been reported.

Class I antiarrhythmic agents such as flecainide have local anesthetic and anticholinergic properties which may rarely cause urinary retention.


A case of reversible flecainide-induced pneumonitis was reported in a 61-year-old man with a remote history of pulmonary tuberculosis. A complete infectious disease work-up was negative. Serial bronchial-alveolar lavages and chest radiographs were consistent with a drug-induced process.

Respiratory side effects are extremely rare.


One patient developed leukopenia after 5 months of flecainide therapy. The leukopenia resolved after drug discontinuation and did not recur when flecainide was reinstituted. The leukopenia may have been due to a concurrent viral infection.

Hematologic side effects are extremely rare.


Hepatic side effects including enzyme concentration elevations have been reported in rare cases.


Psychiatric side effects including paranoid psychosis was reported in a 62-year-old patient receiving flecainide for the treatment of malignant neuropathic pain.

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