What these medications do
Flecainide and propafenone belong to a group we call Class 1c antiarrhythmics. They block sodium channels in heart muscle, which slows how quickly an electrical impulse can travel through the atria. That slowing makes it much harder for atrial fibrillation (AF) or supraventricular tachycardia (SVT) to take hold and keep going. Propafenone has a small extra effect — it weakly blocks beta receptors, similar to a low-dose beta-blocker — but its main action is the same as flecainide.
Because they act mostly on the upper chambers, they are well-suited to AF, atrial flutter, and SVT. They are not first-choice drugs for ventricular arrhythmias.
Who we prescribe them for
We reach for flecainide or propafenone when:
- A patient has paroxysmal AF (episodes that come and go) and we want to either prevent episodes or shorten them when they happen.
- A patient has SVT that is symptomatic but not frequent enough to push toward ablation right away — or as a bridge until ablation.
- A patient has had a successful cardioversion and we want to improve the odds of staying in rhythm.
The single most important rule with these drugs is who we cannot give them to. We do not prescribe flecainide or propafenone to patients with:
- A prior heart attack (MI) or known coronary disease with ischemia.
- Significant left ventricular hypertrophy (thickened heart muscle).
- Reduced ejection fraction or heart failure.
- Severe valve disease.
This restriction comes from the CAST trial in the late 1980s, which showed that giving Class 1c drugs to post-MI patients actually increased death from arrhythmia. The lesson stuck. Before starting either drug, we confirm with an echocardiogram and often a stress test that the heart is structurally normal.
How to take them
Daily prevention dosing:
- Flecainide: typically 50–150 mg twice daily.
- Propafenone: immediate-release 150–300 mg three times daily, or sustained-release 225–425 mg twice daily.
Pill-in-pocket strategy. For selected patients who have infrequent but distinct AF episodes, we may prescribe a single larger dose to take only when an episode starts. The first time we try this, we do it under observation to confirm it converts the rhythm safely. After that, you can use it at home. This approach is not for everyone — it requires reliable symptoms, a structurally normal heart, and tolerance of the test dose.
Both medications are taken with or without food. If you miss a daily dose, take it when you remember unless it’s close to the next one — never double up.
The AV-node blocker pairing
One quirk of Class 1c drugs is that they can convert AF into atrial flutter with a slow flutter rate that the AV node conducts 1:1, sending a very fast signal to the ventricles. To prevent this, we almost always co-prescribe an AV-node-blocking agent — usually a beta-blocker (metoprolol) or a calcium channel blocker (diltiazem, verapamil). You should not take flecainide or propafenone without one unless we have specifically said it is okay.
Side effects to watch for
Most patients tolerate these drugs well. Things we hear about:
- Flecainide: visual disturbances (blurry vision, halos around lights), dizziness, mild nausea, a metallic taste.
- Propafenone: metallic or bitter taste (fairly common), mild fatigue, occasional GI upset.
More serious but rarer: new or worsened arrhythmia (proarrhythmia), worsening heart failure if cardiac function was borderline, and very rarely conduction problems.
Monitoring
After starting, we get an ECG within the first week or two to confirm:
- The QRS is wider than baseline (expected — that tells us the drug is working), but not excessively so.
- The PR interval and QT have not become alarming.
We often do a stress ECG at some point as well — the drugs slow conduction more at faster heart rates, and we want to make sure exercise does not unmask a problem. Periodic ECGs at follow-up are routine.
Drug interactions
Both drugs interact with CYP2D6 and several other enzymes. Drugs that meaningfully raise levels include amiodarone, quinidine, fluoxetine, paroxetine, and ritonavir. Combining with other antiarrhythmics needs care. Always tell us about new prescriptions, and tell other prescribers you are on a rhythm drug.
When to call us
Call if you notice new dizziness or near-fainting, a much faster or more chaotic heartbeat than your usual AF, new shortness of breath, or visual changes that don’t settle.