Lifestyle Modification and Liraglutide

Inclusion Criteria: 1. Participants must have a BMI ≥ 30 and ≤ 55 kg/m² 2. Age ≥ 21 years and ≤ 70 years 3. Eligible female patients will be: * non-pregnant, evidenced by a negative urine dipstick pregnancy test * non-lactating * surgically sterile or postmenopausal, or they will agree to continue to use an accepted method of birth control during the study 4. Ability to provide informed consent before any trial-related activities 5. Participants must: * have a primary care provider (PCP) who is responsible for providing routine care * have a reliable telephone service with which to communicate with study staff * understand and be willing to comply with all study-related procedures and agree to participate in the study by giving written informed consent * plan to remain in the Philadelphia area for the next 18 months Exclusion Criteria: 1. Pregnant or nursing, or plans to become pregnant in the next 18 months, or not using adequate contraceptive measures 2. Personal or family history of medullary thyroid cancer or multiple endocrine neoplasia syndrome type 2 3. Uncontrolled hypertension (systolic blood pressure ≥ 160 mm Hg or diastolic blood pressure ≥ 100 mm Hg) 4. Type 1 diabetes 5. Type 2 diabetes 6. A fasting glucose ≥ 126 mg/dl (on second assessment after first elevated value) 7. Recent history of cardiovascular disease (e.g., myocardial infarction or stroke within the past 6 months), congestive heart failure, or heart block greater than first degree 8. Clinically significant hepatic or renal disease 9. Thyroid disease, not controlled 10. History of malignancy (except for non-melanoma skin cancer) in past 5 years 11. Current major depressive episode, active suicidal ideation, or history of suicide attempts 12. Psychiatric hospitalization within the past 6 months 13. Self-reported alcohol or substance abuse within the past 12 months, including at-risk drinking (current consumption of ≥ 14 alcoholic drinks per week) 14. Use in past 3 months of medications known to induce significant weight loss (i.e., prescription weight loss medications) or weight gain (e.g., chronic use of oral steroids, second generation antipsychotics) 15. Loss of ≥ 10 lb of body weight within the past 3 months 16. History of (or plans for) bariatric surgery 17. Inability to walk 5 blocks comfortably or engage in some other form of aerobic activity (e.g., swimming) 18. Known or suspected allergy to trial medication(s), excipients, or related products 19. Hypersensitivity to liraglutide or any product components 20. The receipt of any investigational drug within 6 months prior to this trial 21. Previous participation in this trial (e.g., randomized and failed to participate) 22. History of pancreatitis 23. Subjects will be included/excluded according to the latest updated US PI. 12-Week Extension Trial: Inclusion Criteria Inclusion criteria are those described for the original 1-year trial (enumerated above). The principal exception from these criteria is that participants will only be required to have a BMI \> 27 kg/m2, with or without co-morbidities, to be eligible to participate in the extension study. All participants will have met BMI inclusion criteria when they initiated the use of liraglutide and now will use it, potentially with phentermine 15 mg/d, to facilitate to the maintenance of lost weight. (Liraglutide is approved for chronic weight management, including following successful weight loss.) We do not wish to enroll participants with a BMI \< 27 kg/m2 because of the possibility that they could reduce substantially below a BMI of 24.9 kg/m2, the upper limit of "normal" weight. Exclusion Criteria: Exclusion criteria will include those listed in the original protocol, including those specific to the use of liraglutide 3.0 mg/d (e.g., family history of medullary thyroid cancer). Additional exclusion criteria added to the 12-week extension study are specific to the use of phentermine 15 mg/d. They include: 1. Use of monoamine oxidase inhibitors in the past 2 weeks 2. Glaucoma 3. Presence or history of marked agitation 4. History of drug abuse 5. Known hypersensitivity to sympathomimetic amines 6. Current use of selective serotonin re-uptake inhibitors (e.g., fluoxetine, sertraline, etc) 7. Current use of any other weight loss medications (besides liraglutide 3.0 mg/d) 8. History of coronary artery disease