Subject has Fragile X Syndrome with a molecular genetic confirmation of the full FMR 1mutation (>200 CGG repeats)
Subject is a male or non-pregnant/non-lactating female aged 6-25 years old, inclusive
Subjects who are capable of becoming pregnant must use an acceptable method of birth control for the duration of the study. These include:
Abstinence (only for subjects who are not sexually active)
Intrauterine devices in place for at least 3 months
Oral contraceptives
Surgical sterilization
Adequate barrier methods (male/female condoms)
Subject must have a parent or caregiver who is willing to participate in the whole study
Subject and/or subject's parent/legal authorized representative is able to understand, read, write and speak English or French fluently to complete study-related materials
For subjects who are not their own legal guardian, parent/legal authorized representative is able to understand and sign informed consent to participate in the study
The use of concomitant medication must be stable, in terms of dose and dosing regimen, for at least 4 weeks prior to screening and must remain stable during the period between first visit (Screening) and the commencement of the study. Every effort should be made to maintain stable regimens of allowed concomitant of the double-blind study medication until the last study assessment
Behavioural/educational treatments must be stable for 4 weeks prior to the first visit (Screening) and must remain stable for the duration of the randomized double-blind study medication
Overall IQ, assessed at Screening on the Leiter-III, is not higher than 79, and subject must speak at least occasional 3-word phrases.
Exclusion Criteria
Inability for subject or parent/caregiver to cooperate or follow through with demands of the study
Subject has a life-threatening medical problem or other major systemic illness that compromises health or safety and/or would interfere with the study
Aged younger than 6 or older than 25 years of age
History of intolerable adverse events with Metformin
Current or recent treatment using metformin (within the past year)
BMI less than 2 standard deviations for age using the World Health Organization scale
Serum creatinine is >1.4 mg/dl (female) or >1.5 mg/dl (male)
Vitamin B12 deficiency
Pregnancy at screening or unwillingness to use acceptable method of birth control, if applicable