Iatrogenic Methotrexate Toxicity Leading to Acute Oral Mucositis: A Rare Case Report

Methotrexate (MTX) is a folate antagonist extensively employed in oral metronomic chemotherapy and the management of various autoimmune disorders. Despite its established therapeutic efficacy, its narrow therapeutic index predisposes patients to severe adverse effects following dosing errors or inappropriate administration. Oral mucositis represents one of the most debilitating manifestations of MTX toxicity and may be accompanied by significant haematological and cutaneous complications. This report describes a rare case of iatrogenic MTX toxicity in a 56-year-old male who inadvertently consumed MTX 7.5 mg daily instead of the prescribed mirtazapine for 10 consecutive days. The patient presented with extensive painful pan-oral ulcerations, purulent discharge, haemorrhagic crusting of the lips, de-papillated erythematous tongue, restricted mouth opening, and multiple necrotic cutaneous lesions involving the extremities and trunk. Haematological evaluation revealed severe anaemia, while peripheral blood smear, exfoliative cytology, and histopathological examination demonstrated features consistent with MTX-induced cytotoxic mucosal injury accompanied by chronic inflammatory cell infiltration. Following prompt recognition of the medication error, MTX was discontinued and supportive management comprising folic acid supplementation, haematinic therapy, topical corticosteroids and anti-inflammatory oral rinses was instituted. Progressive clinical improvement was observed within two weeks, culminating in complete resolution of oral and cutaneous lesions with normalisation of haematological parameters at two months. The present case highlights the potential for severe mucocutaneous and haematological toxicity even with short-term inadvertent exposure to low-dose MTX. Furthermore, it underscores the critical importance of medication reconciliation, patient education, attentive clinical observation and early recognition of oral manifestations to prevent avoidable iatrogenic morbidity and improve medical safety