This study seeks to determine if modifying the hormonal milieu of the menstrual cycle, through administration of exogenous progesterone, will improve the effectiveness of treatments for smoking cessation in women. Progesterone, a gonadal hormone, is used clinically for treatment of endometrial hyperplasia, amenorrhea, dysfunctional uterine bleeding, and for assisted reproduction in women. Progesterone also shows promise for the treatment of multiple central nervous system disorders including cocaine addiction, seizure disorder, and traumatic brain injury. As the next step, the investigators seek to determine if progesterone augments standard smoking cessation treatments (e.g., NRT) in regularly cycling women. The investigators hypothesize that co-treatment with progesterone, compared to placebo, will enhance the effectiveness NRT for smoking cessation. To test this hypothesis, the investigators propose an 8-week, double-blind, placebo-controlled clinical trial, which will randomize 50 smokers using a 1:1 assignment ratio to 400 mg/day progesterone or placebo. Consistent with the Clinical Practice Guidelines, all participants will also receive transdermal nicotine patch (TNP) plus brief counseling for smoking cessation during the study participation.