An antimetabolite is a type of drug that interferes with the normal metabolism of cells, particularly in rapidly dividing cells like those found in tumors. These substances mimic the structure of natural metabolites, which are essential for cell growth and division. By doing so, antimetabolites disrupt the synthesis of DNA and RNA, ultimately inhibiting cell proliferation. Commonly used in cancer treatment, antimetabolites can target specific phases of the cell cycle. Examples include methotrexate and 5-fluorouracil, which are often employed in chemotherapy regimens. Their effectiveness lies in their ability to selectively attack cancer cells while minimizing damage to normal cells.