BackgroundMexiletine, a class 1b antiarrhythmic medication, appears to have some potential for treating muscle stiffness and other symptoms of myotonias.PurposeThe aim of this study was to analyse the effect and safety of mexiletine on myotonia signs and symptoms in patients with myotonic disorders.Material and methodsA retrospective, observational study including all patients treated with mexiletine at the hospital was carried out.Demographic (age and sex), diagnostic (type of myotonic disorder) and therapeutic (dosage, duration of treatment, previous treatment, adverse reactions) variables were gathered. Statistical analysis of the data was carried out using Microsoft Excel.Results11 patients (10 men and 1 woman, aged 40 (21–56) years) were included from May 2011 to October 2015 (1 patient affected by Schwartz Jampel syndrome, 6 affected by Steinert disease, 1 patient with Thomsen disease and 3 patients with Becker muscular dystrophy).7/11 patients (64%) were taking fenitoine, carbamazepine and/or diuretics before starting mexiletine, with no improvement in their clinical symptoms which led to medication interruption.7/11 patients (64%) are still receiving mexiletine treatment (from 2011, 2012 or 2014). They started treatment at a low dose (100 mg/8–12 h) showing null or insufficient benefits. This dose was increased until achieving a final dose of 200 mg/8 h in all of these patients. All reported experiencing good relief of muscle stiffness in response to mexiletine.4/11 patients (36%) stopped the treatment because they presented low or no improvement in their symptoms. They were treated with doses of 100 mg/8 h or 100 mg/12 h. These doses could not be increased due to patient cardiovascular pathology.91% of patients did not present with any adverse effect. Only one adverse effect (mild upper gastrointestinal pain which disappeared in a few days without interrupting the treatment) was reported in one patient.Conclusion64% of patients treated with mexiletine (all at a dose of 200 mg/8 h) showed improvement in their symptoms and are still under treatment.Mexiletine was well tolerated in all patients, with minor adverse effects in only one patient.Due to the fact that these disorders are rare, the number of patients analysed was low.References and/or AcknowledgementsSpecial acknowledgements to the pharmacy service of my hospital.No conflict of interest.