In this 4-day study, participants will be randomly assigned to receive a daily dose of 200 mg of progesterone, 400 mg of progesterone, or placebo. All participants will abstain from smoking for Days 1 through 3. On Day 4, participants will take part in a smoking session, during which their smoking habits, including the number of puffs, duration of puffs, and number of cigarettes smoked, will be measured. Outcome measurements will include self-reports of cravings; carbon monoxide monitoring; and standardized questionnaires to assess nicotine withdrawal symptoms.