Calcium Channel Blocker in Myotonic Dystrophy Type 1

This is a Phase 1 clinical trial designed to evaluate the safety and tolerability of amlodipine, a calcium channel blocker, in adults with Myotonic Dystrophy Type 1 (DM1). Amlodipine is being studied to see if it can improve muscle strength, reduce stiffness (myotonia), and improve function by modifying calcium flow in muscle cells. All participants will receive amlodipine starting at 2.5 mg daily for 2 weeks, then 5 mg for 4 weeks. After that, participants will be randomly assigned to continue on 5 mg or increase to 10 mg for an additional 4 weeks. The main goals are to assess changes in blood pressure and any adverse events to determine whether the drug is safe in this population. The study will also explore how amlodipine affects muscle strength, mobility, fatigue, and daily function using clinical tests and questionnaires. Findings will inform a future phase 2 trial.