Inclusion Criteria:  adults (≥ 18years), genetically confirmed non-dystrophic myotonia (NDM), presence of symptomatic myotonia as reported by the participant.  Exclusion Criteria:  concomitant medical conditions that would preclude the use of mexiletine or lamotrigine, evidence of severe kidney disease or severe liver impairment [estimated glomerular filtration rate (eGFR) as calculated by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) Creatinine Equation <60ml/min/1.73m2], pregnant or breastfeeding women, participation in other treatment studies <30days before enrolment.