Paroxetine is a selective serotonin reuptake inhibitor (SSRI). It is used to treat several diseases, including major depressive disorder, obsessive-compulsive disorder, post-traumatic stress disorder, generalized anxiety disorder, and premenstrual dysphoric disorder. Anaesthesia or surgery may exacerbate the cardiac effects of the drug. Its interaction with anaesthetic drugs may also have an effect on the heart rhythm. In this case report, the mechanism of such an arrhythmia is elucidated upon. We report a case of acquired QT prolongation in a patient receiving paroxetine who underwent lumbar spinal fusion surgery under general anaesthesia. The potential mechanisms, perioperative implications, and management strategies for acquired QT prolongation are discussed.