Atypical Vesicular Skin Eruption Following a Low-Dose Methotrexate Dosing Error: A Case Report.

Methotrexate is widely used to manage various autoimmune conditions, including psoriasis and psoriatic arthritis. Although weekly low-dose regimens are generally well-tolerated, dosing errors, particularly daily instead of weekly administration, can result in acute methotrexate toxicity with mucocutaneous and hematologic complications. We present a case of clinically significant methotrexate toxicity manifesting with painful oral mucositis and vesicular cutaneous lesions on the right lateral abdomen and right supraclavicular region in a 68-year-old Caucasian female patient with psoriasis and psoriatic arthritis after inadvertently taking 15 mg of methotrexate daily for five consecutive days (75 mg cumulative) instead of her prescribed weekly 15 mg dose. The patient was admitted for intravenous hydration with sodium bicarbonate and leucovorin 200 mg every six hours, then transitioned to oral leucovorin for an additional 48 hours on discharge. Mucosal symptoms gradually resolved during her four-day admission, no new skin lesions developed after hospital day 2, and laboratory values trended toward normalization without clinically significant myelosuppression, neutropenia, thrombocytopenia, or hepatorenal injury. This case illustrates that an inadvertent daily-instead-of-weekly dosing error can produce clinically significant mucocutaneous toxicity, and that prompt recognition with leucovorin rescue may help limit progression to the myelosuppression and systemic complications that can accompany methotrexate toxicity. It is important for clinicians to remain vigilant for toxic adverse effects to prevent progression to life-threatening complications and enhance patient education to avoid dosing errors.