A 57 year old woman had been treated for several years with diclofenac 150 mg/day, atenolol 50 mg/day, and triamterene/ hydrochlorthiazide 50/25 mg/day for rheumatoid arthritis (RA) and hypertension. Her doctor started treatment with methotrexate 5 mg/week for active RA. At the start of methotrexate treatment her full blood count was normal except for a haemoglobin of 10.1 g/dl. Renal and liver function were not checked. She weighed 68.5 kg. After one month her haemoglobin had fallen to 8.0 g/dl and her white cell count to 3.6 × 10⁹/l and she developed mouth ulcers, dyspnoea, and easy bruising. She continued taking methotrexate, but after a month of these symptoms was admitted to hospital as an emergency.

On examination she was pale, tachypnoeic, and dehydrated with extensive mucosal ulceration. Investigations …