New study tests safer version of muscle stiffness drug

This study tests a new extended-release form of mexiletine (mexiletine PR) against the standard immediate-release version in people with non-dystrophic myotonias, a group of rare genetic disorders that cause muscle stiffness. About 24 adults aged 16 and older will take each version for 12 weeks to see which is safer and better tolerated. The goal is to find a treatment that controls symptoms with fewer side effects.