RESUMEN
INTRODUCTION: Lymphopenia is a known side effect of dimethyl fumarate (DMF), a disease-modifying therapy (DMT) for patients with multiple sclerosis (pwMS). A body mass index ≥ 30 kg/m2 has been identified as a protective factor; however, no data are available on lymphopenia in pwMS undergoing to weight loss due to bariatric surgery. METHODS: We described two pwMS with history of bariatric surgery who started DMF as DMT. RESULTS: The two pwMS experienced persistent lymphopenia during DMF-treatment, which was resolved after its discontinuation. CONCLUSIONS: Several mechanisms might modify DMF pharmacokinetic profiles after bariatric surgery and its bioavailability. Absolute lymphocyte count should be monitored in pwMS treated with DMF and history of bariatric surgery and weight loss.