Skipping or changing your dose without approval from your doctor may cause your abnormal heartbeat mexiletine on line to return. If the victim has collapsed or is not breathing, call local emergency services at 911. If you are having surgery, including dental surgery, tell the doctor or dentist that you are taking mexiletine. In eight patients with moderate to severe liver disease, the mean half-life was approximately 25 hours. Less common side effects may include sleep problems, weakness, tiredness, blurred vision, dry mouth, headache, diarrhea, constipation, and mild speaking difficulties. Before using this medicine, consult your doctor or pharmacist if you have: certain heart rhythm problems second- or third-degree atrioventricular block unless you have pacemaker shock due to the heart not pumping well.