A folic acid antagonist is a type of medication that inhibits the action of folic acid, a vitamin essential for DNA synthesis and cell division. These drugs are often used in cancer treatment because they can slow down the growth of rapidly dividing cells, such as cancer cells. By blocking folic acid, these medications disrupt the production of nucleic acids, which are vital for cell replication. Common examples of folic acid antagonists include methotrexate and pyrimethamine. While effective in treating certain conditions, they can also lead to side effects due to their impact on normal, healthy cells that also require folic acid for growth and maintenance. Therefore, careful monitoring is essential during treatment.