This pilot feasibility trial is a Phase IV, 12-week, single center, randomized, double blind, parallel group comparison of low (0.5 mg twice daily with slower titration over one full week) versus standard dose (1.0 mg twice daily with standard titration) varenicline in individuals with DSM-V Bipolar Disorder (BD) or Schizophrenia Spectrum Disorders (SSD) with a 12-week, post-treatment follow-up. The 16 visits after screening (Weeks 0-24) include 11 in-person (with medications dispensed at weeks 0, 2, 4, 6, 8 and 10\] and 5 via telephone. Plasma will be obtained at baseline to measure participants' Nicotine Metabolite Ratio (NMR) and to identify slow versus normal nicotine metabolisers. A flexible quit date (between days 8-35) will be employed allowing varenicline preloading to occur prior to the Target Quit Date. Ten sessions of Acceptance \& Commitment Therapy (ACT) for smoking cessation will be delivered by trained counselors.
