Metronidazole has been approved to treat infection. Little is known about the cases of drug-induced optic neuropathy. Literature does not clearly indicate the symptoms, frequency and prognosis of drug-induced optic neuropathy, but there were several case reports of visual impairment caused by metronidazole including visual field defect, myopia, photophobia, optic neuritis and oculogyric crises. This case is a 70-year-old female with a 10-year history of type 2 diabetes, and proliferative diabetic retinopathy. The patient received metronidazole for treatment of her leukorrhea. She had been noted bilateral visual impairment and this became progressively worse even occurrence of sudden blindness. Symptoms began 10 days after initiation of metronidazole therapy. Ophthalmic and neurological examinations were performed to determine the cause of her blindness, and test results favored drug-induced toxic optic neuropathy as the cause of permanent blindness of this patient.