Approximately 20-40% of patients with ulcerative colitis (UC) are obese. The investigators have demonstrated that obesity adversely impacts disease course in patients with UC, leading to higher risk of persistently active disease, surgery, hospitalization, and treatment failure, particularly in biologic-treated patients. Intentional weight loss is effective in improving disease outcomes in patients with inflammatory arthritis, but there is limited data on its impact in UC. While dietary interventions for weight loss have limited efficacy and endoscopic bariatric interventions may be too invasive in patients with UC with active gastrointestinal symptoms, pharmacological weight loss with a highly effective oral agent may be a novel strategy to induce weight loss and augment the efficacy of biologic therapy in UC. Hence, the investigators are conducting a pilot, phase 2A, 22-week, randomized, placebo-controlled clinical trial of phentermine-topiramate in obese patients with active UC starting on a new biologic agent (infliximab, adalimumab, golimumab, vedolizumab). The overall objective is to (1) evaluate the efficacy, safety and tolerability of phentermine-topiramate, and (2) to assess the impact of pharmacological weight loss on clinical outcomes, inflammatory burden and biologic trough concentration in patients with UC. The central hypothesis is that phentermine-topiramate will be safe, effective, and well tolerated in patients with UC, and weight loss would achieve higher rates of clinical and biochemical remission, and higher biologic trough concentration.