This effect appears to be dose dependent, favoring lower doses (i.e., 25 mg daily). Thus, the proposed study seeks to conduct a pilot clinical trial of low dose naltrexone (25 mg daily) compared to placebo for minimizing weight gain in combination with varenicline for smoking cessation. Forty individuals who smoke at least 10 cigarettes per day will receive open-label varenicline for 12 weeks according to the recommended titration schedule up to 1 mg varenicline twice daily. Subjects will be randomized to receive either placebo or 25 mg naltrexone daily, with treatment starting at the quit date (after 1 week on varenicline to minimize nausea, a side effect of both varenicline and naltrexone) and continuing for 11 weeks. Subjects will take 12.5 mg naltrexone daily for the first week and 25 mg naltrexone daily for the next 10 weeks of treatment. In an effort to uncover mechanisms for naltrexone's weight suppressant effects, an experiment will be conducted using food odors and food consumption to examine naltrexone's effects on palatability, incentive value, and alliesthesia.