A Clinical Trial Evaluating Fecal Microbiota Transplantation (FMT) in Adolescents With ADHD

Inclusion Criteria: 1. Between 13-17 years of age with consent of a legal guardian: Participants should be at least 13 years old and not older than 17 years at the day of screening (V1). 2. Have a primary diagnosis of ADHD as confirmed by the Mini-International Neuropsychiatric Interview for Children and Adolescents (MINI-KID). 3. Be on a stable appropriate dose of an appropriate first-line pharmacological treatment for at least 8 weeks prior to the day of screening (V1). a. First line pharmacotherapy treatment will be defined based on the CADDRA guidelines \[63\] and include the following Amphetamine-based psychostimulants: i. Mixed amphetamine salts (amphetamine and dextroamphetamine) ii. Lisdexamfetamine dimesylate Methylphenidate-based psychostimulants: i. Methylphenidate hydrochloride, Methylphenidate hydrochloride (extended release, multilayer release capsules) ii. Methylphenidate hydrochloride (extended release, OROS tablets) iii. Methylphenidate hydrochloride (controlled release, multi-layer beat capsules) iv. Methylphenidate hydrochloride (extended-release oral suspension) 4. Have a score of ≥ 18 on the inattention subset (questions 1-9) and/or the hyperactivity/impulsivity subset (questions 10-18) of the SNAP-IV 26-Item Parent Rating Scale on the day of screening (V1) and the baseline visit (V2). 5. Able to communicate and complete study assessments in English. 6. Able to comply with all protocol procedures. 7. Consenting guardian Exclusion Criteria: 1. Participant meets Diagnostic and Statistical Manual of Mental Disorders (DSM-5) Criteria for the following conditions according to the MINI-KID: 1. Diagnosis of a Substance Use Disorder within the last 3 months prior to screening. \*(Criteria should include Alcohol and Non-Alcohol substances except Cannabis) 2. Moderate or severe Substance Use Disorder for Cannabis use in the last 3 months 3. Currently active high suicidality. Eligibility of Participants who meet criteria for moderate suicidality is determined by clinical judgment of Principal Investigator. 4. Active Anorexia Nervosa or Bulimia Nervosa in the last 3 months. 5. Tic Disorders 6. Psychosis 7. Obsessive Compulsive Disorder 8. Bipolar Disorder 9. Conduct Disorder 2. Participant has a score of ≥ 8 on the oppositional defiant subset of the SNAP-IV 26-Item Parent Rating Scale (questions 19-26) on the day of screening (V1). 3. Intellectual or learning disability based on previous documented diagnosis or clinical judgment of Principal Investigator. 4. Documented diagnosis of Pediatric Acute-onset Neuropsychiatric Syndrome (PANS) or Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections (PANDAS). 5. Documented diagnosis of schizophrenia or schizoaffective disorder. 6. Documented diagnosis of Autism Spectrum Disorder (ASD) or currently undergoing assessment for suspected ASD. 7. Use of systemic antibiotics for medical purposes within the last 3 months prior to the day of screening (V1). 8. Use of prebiotics or probiotics for medical purposes for more than 2 weeks within the last 3 months prior to the day of screening (V1). a) Eligibility and required washout period of participants with use of over-the-counter prebiotics or probiotics will be determined by clinical judgment of Principal Investigator. 9. Use of experimental drugs in the last 3 months prior to the day of screening (V1). 10. Documented clinical diagnosis of inflammatory bowel disease (IBD), Crohn's disease, ulcerative colitis, and/or celiac disease. 11. Documented diagnosis of conditions causing immunosuppression and/or currently receiving immunosuppressive treatments. 12. Documented clinical diagnosis of significant bleeding disorders. 13. History of oropharyngeal dysphagia or other swallowing disorder, and/or self or study partner reported difficulty with taking oral capsules or pills. 14. Breastfeeding, pregnant or seeking to get pregnant during the course of this study. Female participants of childbearing age should be using an acceptable method of birth control (implants, injectable, combined oral contraceptives, IUDs, barrier contraceptives, sexual abstinence, or a vasectomized partner) for the duration of their participation in the trial. 15. Participants who are currently hospitalized or institutionalized. 16. Reported allergy to Vancomycin or Nitazoxanide 17. Hepatic dysfunction: A) Documented history or current diagnosis of an acute or chronic hepatic disease (e.g., cirrhosis, hepatitis, hepatic impairment) OR B) Abnormal - Liver Function Tests (LFTs): Screening laboratory results indicating clinically significant hepatic dysfunction: * Alanine Aminotransferase (ALT) or Aspartate Aminotransferase (AST) ≥3 the Upper Normal Limit (UNL) * Total Bilirubin \> 1.5 × ULN (except in cases of documented Gilbert's Syndrome) 19. Renal dysfunction: A) Diagnosed Renal Disease: Any documented medical history or current diagnosis of kidney disease, acute kidney injury, or other clinically significant renal impairment. OR B) Abnormal Renal Function Tests: Screening laboratory results indicating significant renal dysfunction. Creatinine \> 1.5 × ULN\* * Potential participants presenting with mild, non-clinically significant laboratory abnormalities (e.g., AST/ALT between 1.0 and 3.0 × ULN, or isolated borderline creatinine variations confirmation of enrollment into the study will be dependent of the study physician.