The study utilizes a 3-arm placebo-controlled RCT to study the effectiveness of ketamine in reducing chronic post-mastectomy pain. Participants randomized to the first arm will receive a 0.35 mg/kg dose after induction, followed by a 0.25 mg/kg/hr infusion during surgery (up to a maximum of 6 hours) and continued for 2 hours postoperatively. Participants in the second arm will receive a single dose of 0.6 mg/kg of ketamine in the post-anesthesia care unit, and the final group will serve as the control group and receive saline (no ketamine).