Mexiletine is a medication closely related to lidocaine that can be taken by mouth (instead of being injected). Mexiletine stops the type of sodium currents that are thought to cause muscle cramps. Mexiletine is a relatively older medication that has been extensively studied in humans. It has been shown to reduce the electrical measures of muscle cramps for other disease conditions. For example, in patients with another severe nerve disease - Machado-Joseph disease (SCA3) - mexiletine treatment led to a decrease in the average number of muscle cramps from 24 to 3 cramps per month.. The safety profile of mexiletine is good, with the most frequent side effects being nausea or other abdominal symptoms. These side effects are rare at the doses (300 mg/day) used in this study. In patients with normal heart function, mexiletine has a minimal effect on heart rhythm. In previous clinical trials, no subject developed any serious heart rate problem.