Inclusion Criteria:  Subject and caregiver are both able to understand English clearly and caregiver can read and write English to complete study assessments Males aged 13-40 years (inclusive) Has FXS with molecular genetic confirmation of the full FMR1 mutation (>200 CGG repeats). Weight ≥45 kg Is in general good health as deemed by the Investigator, determined by physical examination, medical history, and laboratory tests If receiving sertraline, is on a stable, well-tolerated dose for the previous 3 months with no further changes anticipated  Exclusion Criteria:  Active or history of peptic or gastric ulcer or hemorrhage Any chronic major medical comorbid condition deemed by the Investigator as presenting added risk to the subject, including but not limited to refractory hypertension, kidney disease, or liver disease Diagnosed with diabetes (Type 1 or II) or receiving any anti-diabetic medication Unstable seizure disorder defined by any seizure within 6 months prior to baseline visit and/or a change in any anti-convulsant drug dosing in the 60 days prior to study entry Individual or family history of myocardial infarction, TIA, peripheral arterial disease, stroke or other significant cardiovascular disease Chronic use of NSAIDs or other anti-inflammatory agents Currently taking or have taken any cannabidiol (CBD) preparation within 30 days prior to randomization Currently taking or have taken sulindac within 30 days prior to randomization Changes in chronic psychotropic or anti-convulsant (where taken for reasons other than seizure control) drug treatment within 30 days prior to randomization History of adverse effects of sulindac or other NSAIDS that would prevent safe study completion Unable or unwilling to take oral medication (whole capsule) or history of dysphagia or malabsorption Has current or suspected symptoms of COVID-19 including, but not limited to fever, dry cough, new loss or change of smell or taste, myalgia, fatigue, or a positive COVID-19 test. Has abnormal baseline laboratory assessments including, but not limited to alanine aminotransferase (ALT) or aspartate aminotransferase (AST) >1.5 × the upper limit of normal (ULN), total bilirubin or creatinine >1.0 × ULN, or other clinically relevant laboratory abnormality Has a clinically significant abnormal electrocardiogram (ECG), heart rate, or blood pressure at screening as judged by the Investigator Has received an investigational drug (either approved or not approved) in any prior clinical study within 30 days or 5 half-lives (whichever is longer) prior to screening