5-Fluorouracil (5-FU)-Induced Sinus Bradycardia: A Case Report

5-fluorouracil (5-FU) is the backbone of the majority of approved chemotherapy regimens for colorectal cancer. Common adverse events of 5-FU include diarrhea, nausea, mucositis, and bone marrow suppression. Cardiotoxicity is not common. Sinus bradycardia induced by 5-FU is a rare complication but can be fatal if not managed well. We present a 79-year-old woman with colon cancer who developed bradycardia while receiving 5-FU infusion. The arrhythmia returned to sinus rhythm after cessation of 5-FU infusion. The mechanism of 5-FU-induced cardiotoxicity is unclear and there are no prophylactic medications that can prevent this side effect. If symptoms and signs of cardiovascular anomalies develop, infusion should be stopped immediately and a work-up performed. Rechallenging with a lower dose of 5-FU, shifting to an oral prodrug of 5-FU, or replacing 5-FU with raltitrexed should be considered based on risk-benefit assessment.