To understand if Mexiletine will restores normal colonic motility in a patient with irritable bowel syndrome - constipation (IBS-C) and a genetic mutation in SCN5A.

To understand if Mexiletine will restore normal colonic motility in a patient with a missense mutation in SCN5A, which encodes a voltage-sensitive sodium channel NaV1.5 as it is known to rescue certain expression defects of NaV1.5 in vitro.

Inclusion Criteria:

• Established IBS diagnosis with a known A997T mutation in NaV1.5T

Exclusion Criteria:

• Normal 48-hour baseline transit study
• Current use of mexiletine
• Inability to stay in Rochester for 9 days for testing
• Known allergy to mexiletine
• Inability to complete daily diary
• Taking a medication known to affect myotonia, had a coexisting neuromuscular disease, or had another serious medical illness including second- or third-degree heart block, atrial flutter, atrial fibrillation, ventricular arrhythmia, history of cardiac arrhythmia requiring medication, congestive heart failure, symptomatic cardiomyopathy, or symptomatic coronary artery disease
• Inability to withdraw medications known to interact with mexiletine. Patient is currently not on any of these medications