Quinidine is a cinchona alkaloid consisting of cinchonine with the hydrogen at the 6-position of the quinoline ring substituted by methoxy. It has a role as an alpha-adrenergic antagonist, an antimalarial, an anti-arrhythmia drug, a sodium channel blocker, a muscarinic antagonist, a P450 inhibitor, a potassium channel blocker, an EC 3.6.3.44 (xenobiotic-transporting ATPase) inhibitor, an EC 1.14.13.181 (13-deoxydaunorubicin hydroxylase) inhibitor and a drug allergen. Quinidine, a useful antiarrhythmic compound, is usually contaminated with dihydroquinidine, a compound that itself shows potent antiarrhythmic activity.