Chemotherapy drug. Computer model of a molecule of the chemotherapy drug fluorouracil (5FU). Atoms are represented as rods and are colour-coded: carbon (yellow), hydrogen (white), oxygen (red), nitrogen (blue) and fluorine (green). Fluorouracil is used to treat bowel, breast and stomach cancers. It belongs to a family of drugs known as antimetabolites. These drugs closely resemble a substance, or metabolite, that is essential for a cell's normal biochemical reactions. By replacing the metabolite, the drug prevents the cells from growing or reproducing.