Urine Discoloration Associated With Metronidazole: A Rare Occurrence

Objective: To report a case of metronidazole-induced urine discoloration in a patient with Clostridium difficile sepsis. Case Summary: A 52-year old man was admitted with sepsis secondary to C difficile colitis, which developed after he had been recently treated with broad-spectrum antibiotics for community-acquired pneumonia. The C difficile infection was treated with metronidazole, and the patient subsequently developed cola-colored urine. When metronidazole was inadvertently stopped for 34 hours, the urine color returned to normal, but again darkened when the medication was restarted. The patient suffered no clinically adverse effects from the abnormal urine color. He completed the treatment course for colitis and was discharged to home. Discussion: Urine discoloration is a known side of metronidazole. However, it has been poorly reported in the literature, and many clinicians are unaware that it may happen. Here we report the case of a patient who developed dark urine while receiving treatment with metronidazole. Other potential causes of the urine discoloration were explored, including hemolysis, rhabdomyolysis, or adverse reactions to other medications, with no clear positive findings. An objective causality assessment (Naranjo probability scale) revealed that the urine discoloration was probably due to metronidazole. Conclusions: Metronidazole can cause urine discoloration without otherwise harming the patient. Clinicians should be aware of this potential side effect and provide reassurance to patients who develop abnormal urine that there are no clinically relevant adverse outcomes.