Participation in this study will last 9 weeks and will involve two separate 4-week treatment periods, with a 1-week washout period between them. During the first treatment period, participants will be randomly assigned to receive either mexiletine or placebo, both of which will be taken three times a day. This will be followed by 1 week of no treatment. During the second treatment period, participants will receive whichever treatment they did not receive initially and will follow the same dosing schedule.