A 57-year-old shepherd was referred with a 2-week history of decreased visual acuity in both eyes. Optical coherence tomography (OCT) (Heidelberg Engineering GmbH, Heidelberg, Germany) revealed diffuse outer nuclear layer hyperreflectivity and indistinguishable external limiting membrane and ellipsoid zone. The patient announced to us that he took two 500 mg of closantel tablets (15.15 mg/kg) three days before the initiation of visual problems for sore throat as an antibiotic. Electroretinography displayed severely attenuated responses in both eyes. We decided to admit the patient with the presumed diagnosis of closantel retinal toxicity and treated him with intravenous methylprednisolone 1 g per day and intravenous erythropoietin 10000 IU twice a day, and reevaluation of the patient proved no change in his visual acuity on the third day of admission. Closantel is a veterinary drug with serious side effects in the human retina and central nervous system even in previously reported doses. Public awareness and appropriate drug labeling about its side effects could prevent accidental toxicity. OCT is a noninvasive and rapid diagnostic modality that should be done in suspected toxic retinopathy.