Weight reduction is the most important treatment target in obese women with polycystic ovary syndrome (PCOS), yet it is usually hardly achievable with lifestyle intervention alone. Glucagon-like peptide 1 (GLP-1) receptor agonist (RA) liraglutide was recently approved as an anti-obesity drug but with some limitations, which include high cost and the lack of long-term efficacy and safety data regarding weight reduction. In addition, weight loss achieved with liraglutide is often non-sustainable after treatment cessation. Although DPP-4 inhibitors are weight neutral, they reduced weight regain in animals previously treated with GLP-1 RAs if they were switched to DPP-4 inhibitor rather than placebo. The aim is to evaluate whether DPP-4 inhibitor sitagliptin in combination with metformin reduce body weight regain more effectively than metformin alone in obese PCOS who had been previously treated with liraglutide.