Varenicline is the best smoking cessation agent to date; however it is only effective in a subgroup of smokers and is associated with undesirable side effects in other subgroups. To understand the underlying pharmaco-heterogeneity, the proposed project will use perfusion functional magnetic resonance imaging and a functional candidate gene association approach using brain, behavioral, and clinical endpoints in a placebo-controlled study of chronic varenicline administration in smokers. Brain and behavioral responses to smoking cues will be will be significantly greater in 9/10-repeats compared to 10/10-repeats. DAT 9/10-repeat smokers receiving varenicline will have better treatment outcome compared to 10/10-repeats. For the purposes of the clinical trial portion of the study, the change from cigarettes per day at Baseline to the last day of treatment will be reported.