STRENGTH Study: Supplement Treatment Evaluation of L-carnitine for Muscle Fatigue and Weakness in Children with Neurofibromatosis Type 1

All participants will be prescribed a daily dose of L-carnitine equivalent to the nearest 500 mg dose of 50 mg/kg/day or placebo, given as a split dose to be taken at breakfast and at dinner. L-carnitine can either be given as hard capsules (500 mg) or as a powder for those seeking to avoid taking larger numbers of capsules. Participants will visit the study centres every 6 weeks during the intervention period for 12 weeks, then have a one-week washout. After Week 12 visit eligible patients will move on to the crossover study. This will be a further 12-week Treatment/Supplementation period. The purpose of study visits Week 6 and Week 12 is to undertake outcome assessments, complete patient and parent questionnaires and to provide more supplements to patients. Crossover period is when patients switch over between intervention arm and placebo arm. To monitor patient's adherence to the intervention a dosing diary will be provided to participants/guardians every study visits along with the IP tablets when they are dispensed. This will include date and time IP was taken by the patient and reasons if they were missed. Also, a robust process of counting leftover IP upon return will be done by the trial pharmacists and nurses and will be recorded on the patients' and pharmacy files.