Quinidine is a class I antiarrhythmic drug and is used for the treatment of atrial fibrillation to maintain normal sinus rhythm.

Quinidine blocks cardiac sodium channels and depress phase 0 of the action potential. Side effects include prolonged QT interval causing polymorphic ventricular tachycardia (Torsades de Pointes), cinchonism and thrombotic thrombocytopenic purpura.

Due to the toxicities, quinidine is rarely used in modern clinical medicine.