Inclusion Criteria:  Male aged 18 to 45 years, inclusive. Participant has Fragile X Syndrome with a molecular genetic confirmation of the full Fragile X Mental Retardation (FMR1) mutation (≥200 CGG repeats). Current treatment with no more than 3 prescribed psychotropic medications. Anti-epileptic medications are permitted and are not counted as psychotropic medications if they are used for treatment of seizures. Anti-epileptics for other indications, such as the treatment of mood disorders, count towards the limit of permitted medications. Permitted concomitant psychotropic medications must be at a stable dose and dosing regimen for at least 2 weeks prior to Screening and must remain stable during the period between Screening and the commencement of study medication. Anti-epileptic medications must be at a stable dose and dosing regimen for 12 weeks prior to Screening and must remain stable during the period between Screening and the commencement of study medication. Participants with a history of seizure disorder who are currently receiving treatment with anti-epileptics must have been seizure-free for 3 months preceding screening, or must be seizure-free for 3 years if not currently receiving anti-epileptics. Behavioral and therapy treatments/interventions must be stable for 4 weeks prior to Screening and must remain stable during the period between Screening and the commencement of study medication, and throughout the study. Minor changes in hours or times of therapy that are not considered clinically significant will not be exclusionary. Changes in therapies provided through a school program, due to school vacations, are allowed. Participant must be willing to practice barrier methods of contraception while on study, if sexually active. Abstinence is also considered a reasonable form of birth control in this study population. Participant has a parent, legal authorized guardian or consistent caregiver. Participant and caregiver are able to attend the clinic regularly and reliably. Participant is able to swallow capsules. For participants who are not their own legal guardian, participant's parent/legal authorized guardian is able to understand and sign an informed consent form to participate in the study. If participant is his own legal guardian, he can understand and sign informed consent to participate in the study. If participant is not their own legal guardian, the participant provides assent for participation in the study, if the participant has the cognitive ability to provide assent.  Exclusion Criteria:  History of, or current cardiovascular, renal, hepatic, respiratory, gastrointestinal, psychiatric, neurologic, cerebrovascular, or other systemic disease that would place the participant at risk or potentially interfere with the interpretation of the safety, tolerability, or efficacy of the study medication.  Common diseases such as mild hypertension, well-controlled type 2 diabetes mellitus (hemoglobin A1C [Hgb A1C] <6.5%), etc. are allowed per the investigator's judgment as long as they are stable and controlled by medical therapy that is constant for at least 4 weeks before randomization.  Clinically significant abnormalities, in the investigator's judgment, in safety laboratory tests, vital signs, as measured during Screening. History of substance abuse within the past year, according to investigator assessment. Use of CYP3A4 inhibitors, beta-blockers, MAO inhibitors or triptans at any time during participation in the study. Significant hearing or visual impairment that may affect the participant's ability to complete the test procedures. Concurrent major psychiatric condition (e.g., Major Depressive Disorder, Schizophrenia or Bipolar Disorder) as diagnosed by the investigator. Participants with additional diagnosis of Autism Spectrum Disorder or Anxiety Disorder will be allowed as these are characteristics of FXS. Participant has active diseases that would interfere with participation, such as acquired immunodeficiency disorder, hepatitis C, hepatitis B, or tuberculosis. Participant is planning to commence psychotherapy or cognitive behavior therapy (CBT) during the period of the study or had begun psychotherapy or CBT within 4 weeks prior to Screening. Participant has participated in another clinical trial within the 30 days preceding Screening.