Inclusion Criteria:

• Documented confirmation of Prader-Willi Syndrome (PWS)
• Stable care setting with same, single designated caregiver for at least 6 months prior to Visit 1
• At least 7 years of age or older in the US at the time of consent
• At least 10 years of age or older in Australia
• At least 13 years of age or older in countries outside of the US and Australia

Exclusion Criteria:

• Diagnosis of schizophrenia, bipolar disorder, personality disorder or other severe mood, anxiety or eating disorder (other than hyperphagia).
• Presence of any malignancy within 5 years with the exception of basal or squamous cell carcinoma of the skin, in situ carcinoma of the service, or in situations prostate cancer.
• Presence of clinically relevant renal, hepatic, pancreatic, cardiovascular, neurological, psychiatric, hematological, pulmonary, or GI abnormality that, in the opinion of the investigator, may preclude the patient from safe completion of the study
• Adults: systolic blood pressure >=160 mmHg and/or diastolic blood pressure >=100 mmHg
• Children and Adolescents: systolic blood pressure >=140 mmHg and/or diastolic blood pressure >=90 mmHg.
• Type 1 diabetes mellitus; HbA1c >8.5%
• Use of agents to promote weight gain or loss, alter hunger or appetite within 30 days of Visit 1 and throughout the study.
• Use of any commercially available medication for the treatment of hyperphagia (i.e., Vykat) within 60 days of randomization and throughout the study.
• Very high doses of glucocorticoids in the previous 3 months of Visit 1 and throughout the study.