Study participation will last 6 weeks after subjects enter the study following the screening and physical exam sessions (about 8 weeks total). Participants will engage in two identical study phases, each involving visits over 3 weeks and varying only in whether active drug or placebo is administered. The 3 weeks will involve: baseline visits, dose run-up (week 2), and week 3 (for abstinence assessments). The first week of each period will be a baseline week in which they smoke normally without any medication. During week 2, subjects will begin the dose run-up of active drug or placebo, increasing over 4 days from 1 tablet of 50 mg once daily to 100 mg b.i.d. (two 50 mg tablets twice/day), which will continue through the second and third weeks. Active drug or placebo will be administered in counter-balanced order in a cross-over design. During week 3, subjects will be instructed to try to abstain on each day, i.e. Mon-Friday. Most visits will last 60 mins and involve psychiatric assessments (including psychopathology and neurological and other side effects) ratings, providing an expired breath carbon monoxide (CO) measure that assesses smoking exposure in the past 24 hrs, as well as completing brief self-report measures of craving, withdrawal, and mood. On week 3 of each phase, one visit will involve cognitive testing. On Fri of week 3, subjects will discontinue all medication and resume ad lib smoking prior to the next study period, involving the same 3-week procedure (but with the other medication condition): baseline, dose run-up, full dose administration plus abstinence assessment. Note that active medication will be taken during only one period, with placebo taken during the other period. Pill counts will be used to measure adherence.