Objective:Pancytopenia is an unusual complication of low‐dose methotrexate therapy in rheumatoid arthritis. We report a near fatal case that followed an accidental overdose.Clinical features:An 80‐year‐old Caucasian woman with rheumatoid arthritis presented with pancytopenia and severe mucositis. She had taken her weekly methotrexate dose on four sequential days, due to an error in filling a “Dosette” box.Intervention and outcome:Following treatment with piperacillin, gentamicin and folinic acid, she recovered completely.Conclusions:Low‐dose methotrexate therapy is uncommonly associated with haematological toxicity. This generally occurs in patients with known risk factors for such reactions. Patient reliability should be considered if methotrexate therapy is contemplated.