The primary aim of the study is to determine the safety and tolerability of treatment with Varenicline in methamphetamine-dependent volunteers. The investigators also seek to determine the effects of treatment with Varenicline, as compared to placebo, on craving for methamphetamine or cigarettes following exposure to methamphetamine and smoking cues, respectively. The effects of treatment with Varenicline, as compared to placebo, on subjective effects produced by administration of methamphetamine or placebo will be attempted to be determined. Lastly, the investigators hope to determine the effects of treatment with Varenicline, as compared to placebo, on reinforcing effects produced by administration of methamphetamine or placebo.