1.Stable methadone maintenance patients receiving varenicline will be more likely to maintain abstinence than patients receiving placebo 2.There will be no differences in the type and number of symptoms reported between stable methadone maintenance patients receiving varenicline and placebo 3.There will be no changes in methadone dosage between abstinent and non-abstinent smokers 4.There will be no differences in efficacy, withdrawal symptoms, and safety of varenicline between male and female participants