Methotrexate is an FDA approved drug for treating severe psoriasis. It is associated with toxicity owing to its narrow therapeutic range. Here the patient inappropriately self–administered 40 mg of methotrexate. Following this the patient developed oral mucositis and ulceration within psoriatic plaques. A diagnosis of methotrexate toxicity was made based on clinical findings. The hematological profile was within normal limits. The incidence of ulceration within psoriatic plaques with normal laboratory parameters has been reported very rarely. We report a case where methotrexate toxicity was responsible for ulceration of psoriatic plaques.