Propafenone (proe pa FEEN one) prevents and treats a fast or irregular heartbeat (arrhythmia). It is often used to treat a type of arrhythmia known as AFib (atrial fibrillation). It works by slowing down overactive electric signals in the heart, which stabilizes your heart rhythm. It belongs to a group of medications called antiarrhythmics. This medicine may be used for other purposes; ask your health care provider or pharmacist if you have questions.