Methotrexate. Molecular model of the antimetabolite and antifolate drug methotrexate (C20.H22.N8.O5). This drug is used in the treatment of cancer, autoimmune diseases, ectopic pregnancy, and for the induction of medical abortions. Atoms are represented as spheres and are colour-coded: carbon (grey), hydrogen (white), nitrogen (blue) and oxygen (red).