The purpose of this study is to determine the effects of Nicotinamide Riboside (NR) supplement in adult-onset symptoms of mitochondrial myopathy.

Inclusion Criteria:

• Biochemically and/or genetically confirmed or confirmed primary mitochondrial myopathy based on published diagnostic criteria.
• Biochemically confirmed mitochondrial disorder would mean that the patient meets clinical criteria and has either biopsy or biochemical testing that supports the diagnosis.
• Confirmed mitochondrial disorder means that the patient meets published clinical criteria for the diagnosis and has also had confirmatory genetic testing for the disorder type.
• Agreed to avoid vitamin supplementation or nutritional products with vitamin B3 forms 14 days prior to the enrollment and during the study in order to not exceed 200 mg/day of vitamin B3 derivatives intake.
• Female of childbearing potential agreed to use effective contraception throughout the study.
• Written, informed consent to participate in the study.

Exclusion Criteria:

• Unwilling to comply with the follow-up schedule.
• Inability or refusal to give informed consent by the patient or a legally authorized representative.
• Known pregnancy or breastfeeding.
• Concurrent participation in another investigational drug study or within washout period of treatment.
• Presence of other medical symptoms or condition, which may interfere with interpretation of outcome measures as determined by the study PI.

Clinical / Laboratory Exclusion Criteria:

• Estimated Glomerular Filtration Rate (eGFR) < 30 mL/min.
• Patients in permanent Renal Replacement Therapy.
• Serum alkaline phosphatase 50% above normal limit.
• Serum aspartate transaminase 50% above normal limit.
• Serum Thyroxine (T4) 50% above or below normal limit.
• Serum Thyroid Stimulating Hormone (TSH) 50% above or below normal limit.
• Severe anemia with Hb < 7g/dL.
• Severe leukocytosis with WBC > 15,000/mm^3.