Participation in this study will last approximately 4 weeks. Participants will first attend a screening visit that will include a review of their medical and psychiatric history, a physical examination, vital signs measurements, an electrocardiogram (EKG) test, blood and urine sample collection, an eating questionnaire, and a clinical interview to confirm an ADHD diagnosis. Eligible participants will then be randomly assigned to receive either betahistine or placebo on subsequent study visits.