Attention deficit hyperactivity disorder (ADHD) is a disorder in which problems with paying attention and focusing disrupt people's lives. Betahistine is a drug that activates histamine, a compound found in the human body that regulates immune response and acts as a neurotransmitter. Increased availability of histamine in the brain is known to enhance attention and cognition. In previous studies, betahistine increased vigilance in healthy participants. Presently, betahistine is used outside the United States to treat vertigo, but at doses that are too low to have an effect on attention and cognition. Based on data regarding the use of betahistine outside the United States and on data from other studies using betahistine, there do not appear to be any significant side effects from its use at low doses. This study will determine both whether betahistine is effective in increasing vigilance in people with ADHD and whether it has any side effects at higher doses.

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.

Participants will then attend three weekly study visits, starting 1 week after completing screening. At each visit, participants will receive a dose of either betahistine or placebo. The dose of betahistine will increase at each visit, starting at 50 mg, then increasing to 100 mg, and then to 200 mg. Participants will be asked to refrain from eating and to drink only water in the 12 hours prior to study visits. Also at each visit, participants will have their vital signs measured, complete questionnaires concerning appetite and mood, perform attention and memory tests, provide several blood samples, undergo an EKG test, and perform a breathing test. Some of these tests will be repeated several times over the course of each study visit. Additionally, at the last study visit, participants will undergo a physical examination and provide urine samples.