- Tobacco Use: Current or prior history of at least 10 pack-years of cigarette smoking (e.g., 20 cigarettes/day for 10 years). One pack-year is defined as 20 manufactured cigarettes (1 pack) smoked per day for 1 year. Former smokers are defined as those who have stopped smoking for at least 6 months prior to Visit 1 (Screening and Run-in Visit). Former smokers are eligible to enter the study provided they have at least 10 pack-years smoking history. Subjects making a conscious decision to stop smoking at any time during the study and who refrain from smoking for >4 weeks will be discontinued from the study. Additionally, subjects who start smoking during the study and smoke for at least 7 consecutive days will be discontinued from the study.