A woman in her seventies presented to the accident and emergency department (A&E) with shortness of breath that had increased over a period of three weeks. She had a history of COPD, hypertension and polymyalgia rheumatica. A medication error involving methotrexate, used for autoimmune diseases, was discovered during her medical history review.The patient arrived with stable vital signs, including 94 % oxygen saturation and a respiratory rate of 20 breaths/min. She had been taking 2.5 mg of methotrexate daily for the past three weeks instead of the prescribed weekly dose of 15 mg. Other examinations revealed no alarming findings, except for a slightly elevated D-dimer level.Considering her medical history and exclusion of other differential diagnoses, methotrexate toxicity was suspected. The patient was admitted to the hospital and intravenous folinic acid was initiated as an antidote treatment. Five days later, the patient was discharged with an improvement in the shortness of breath. This case underscores the importance of effective communication in health care, particularly in complex cases like this, where understanding dosages and administration is crucial. Medical history, clinical examinations and medication reviews, often involving clinical pharmacists, are vital in the A&E to reveal medication errors.