Varenicline and placebo will be dispensed in 0.5 mg and 1 mg color coded capsules. During the first 3 days of treatment, participants will take one 0.5 mg tablet of varenicline (or placebo) by mouth daily. If the medication is well-tolerated, the dose will be increased to 0.5 mg by mouth twice daily for 4 days. From day 8, the dose will be increased to the standard dosing schedule of 1 mg (1 tablet) by mouth twice daily. At the end of the 8th week, varenicline will be discontinued.