If your dog is diagnosed with dilated cardiomyopathy, that means his heart isn't pumping blood properly. Abnormal heart rhythm, or arrhythmia, often occurs in dogs with dilated cardiomyopathy. Eventually, dogs with dilated cardiomyopathy develop congestive heart failure. If your dog suffers from ventricular arrhythmia -- abnormal rhythms originating in the heart's lower chamber -- your vet might prescribe mexiletine, marketed under the name Mexitil. While mexiletine can't cure ventricular arrhythmia, it can help reduce contractions.
Before prescribing mexiletine, your vet performs a chest X-ray and electrocardiogram on your dog to examine his heart. She can hear an arrhythmia through a stethoscope. Mexiletine is available in pill form. Give the drug with food, since that helps decrease the incidence of gastric upset, a common side effect. Other side effects include tremors, breathing difficulties and loss of balance. Dogs diagnosed with liver disease, low blood pressure, seizures and certain cardiac issues shouldn't receive mexiletine.