Guanfacine Extended Release for the Reduction of Aggression and Self-injurious Behavior Associated With Prader-Willi Syndrome

Inclusion Criteria: * Diagnosis of PWS confirmed by genetic testing documentation * Rating of moderate or above on the Clinical Global Impression- Severity Scale Exclusion Criteria: * Subjects with positive pregnancy test, swallowing difficulty, and/or presenting with active psychosis or mania will be excluded * Subjects currently taking guanfacine extended release * Patients with lactose intolerance * Individuals with pre-existing, clinically significant bradycardia (\< 8 years: \<64 bpm; 8 to 12 years: \<59 bpm; 12 to 16 years: \<53 bpm) or hypotension, defined as 5th percentile for height and gender,26 will be excluded from the study. * Subjects receiving antipsychotic medications due to a documented history of psychosis or bipolar disorder will be allowed to continue taking the medication without dosage modification. * Growth hormone, thyroid hormone replacement treatment, and non-psychiatric medicines will be allowed to continue. * N-Acetyl Cysteine and anticonvulsant medication (only if prescribed for seizures) will be allowed to continue, with specific instructions to not make any dosage changes during the clinical trial.