The purpose of this research study is to learn about the efficacy of a medication called varenicline (Chantix) in treating ADHD in adults and in reducing cigarette smoking in adults with ADHD. The investigators hypothesize that ADHD symptomatology in adults with ADHD will be improved with varenicline treatment. The investigators also hypothesize that varenicline treatment will result in significant reductions in cigarette smoking. Another objective of this study is to more fully evaluate the response and tolerability to varenicline in this group of cigarette smoking adults with ADHD.