The following case report highlights one of the known Flagyl side effects.
A 68 year old diabetic man was restarted post-operatively on his normal dose of the blood thinner Coumadin (warfarin) for his chronic, intermittent atrial fibrillation. He had received a femoral tibial bypass for severe peripheral vascular disease and gangrene of his first and second toe on the right foot. His post-operative antibiotics included a 10 day course of metronidazole (Flagyl) and cefotetan to provide broad-spectrum coverage for his infected foot. The
patient did well until post-operative day #8, when he was noted by the intern on morning rounds to have severe bilateral flank ecchymosis (bruising) and a drop in hemoglobin to 6gms/dl.
Metronidazole inhibits the specific enzyme in the liver that metabolizes warfarin (CYP 450 2C9). If a patient is on warfarin and metronidazole is added, the warfarin dose should be lowered, or at the very least, the INR should be closely monitored to insure that the patient does not become over anticoagulated and develop a bleeding problem.