Study of Cannabidiol in Sanfilippo Syndrome

Inclusion Criteria: * MPS III diagnosis confirmed by genetic testing * ≥ 4 years of age * Patient or parent/legal guardian is able and willing to provide informed consent. For patients 7 to 16 years of age, assent must also be provided when cognitively possible. * If taking any of the following, no dose changes for the last 8 weeks: 1. Anakinra 2. Fluoxetine 3. Probiotic supplement * One of the following criteria are met: 1. Previous participation in a gene/cell therapy or enzyme restorative clinical trial. 2. Previously ineligible to participate in a gene/cell therapy or enzyme restorative clinical trial due to advanced disease state. 3. Functional age, as measured by the Vineland, is ≤ 0.5 chronological age Exclusion Criteria: Equitable selection will be used when choosing participants. An individual who meets any of the following criteria will be excluded from participation in this study: * Mutation known to cause slowly progressive disease * Taken any form of cannabis, including cannabidiol, in the last 8 weeks * Currently enrolled in another ongoing clinical trial * Concomitant use of any of the following therapies: * Rifampin * Diazepam (except for intermittent use as needed for treatment of a prolonged seizure episode) * Clobazam * Stiripentol * Everolimus, sirolimus, tacrolimus * Digoxin * Valproate * Recreational or medical Tetrahydrocannabinol (THC) or synthetic cannabinoid medications (including Sativex) within the last three months * Felbamate (if taking for less than one year) * Non-pharmacological therapies (e.g. ketogenic diet) must be stable for up to four weeks prior to enrollment * Clinical evidence of liver disease or liver injury as indicated by the presence of abnormal tests (AST or ALT \> 2 x ULN; Bilirubin \> 2 x ULN) * Known hypersensitivity to any components of Epidiolex (cannabidiol) * Pregnant or lactating women * Any other social or medical condition that the investigator believes would pose a significant hazard to the subject if the investigational therapy were initiated or be detrimental to the study