Addition of Antibiotics to Upfront Treatment Regimen for Colorectal Cancer

This study is testing whether adding a specific combination of antibiotics (to modify gut bacteria) to standard chemotherapy improves outcomes for people with stage IV (metastatic) colorectal cancer. Research suggests certain gut bacteria may affect how well chemotherapy works. **You may be eligible if...** - You have been diagnosed with stage IV (advanced, spread) colorectal cancer - You have measurable disease on scans - You are about to start first-line chemotherapy with a standard 5-fluorouracil (5FU)-based regimen - Your blood counts and organ function are within acceptable ranges - You are not pregnant or breastfeeding **You may NOT be eligible if...** - You have had your entire colon removed (total colectomy) - You have Cockayne Syndrome - You are taking certain medications that interact with the study antibiotics (disulfiram, tizanidine, theophylline, or high-dose methotrexate) and cannot stop them - You are allergic to ciprofloxacin, metronidazole, or aspirin - You received a full course of antibiotics in the 30 days before starting chemotherapy - You have an abnormal heart rhythm (prolonged QTc interval) on an ECG - You have a condition that affects your ability to absorb or swallow medications Talk to your doctor to see if this trial is right for you.