The purpose of the study is to compare the effects of L-citrulline and metformin and their combination therapy on muscle function and force in patients with Becker muscular dystrophy (BMD).

This is a single center, not-randomized open study. The study medication consists of L-citrulline and metformin administered orally; 5g citrulline and 500mg metformin tablets will be given 3 times daily.

Half of the patients will be treated with metformin during the first 6 weeks of the study. The other patients will receive initially L-citrulline for 6 weeks, before all patients will be treated with the combination therapy (metformin and L-citrulline) for another 6 week period.

The study lasts 12 weeks and includes the screening visit and 3 further visits at baseline, week 6 and week 12.

Inclusion Criteria:

• 18 years or older
• Molecular or immunohistochemical diagnosis of BMD
• ambulant at the time point of screening

Exclusion Criteria:

• Participation in another therapeutic BMD study within the last 3 months
• Use of L-Arginine, L-Citrulline or Metformin within the last 3 months
• Other chronic disease or relevant limitation of renal, liver, heart function according to discretion of investigator
• known hypersensitivity to L-citrulline or metformin