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.