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coumarin Any of a group of organic compounds found widely in plants and derived from the amino acid phenylalanine. They include coumarin itself, which gives new-mown hay its characteristic fragrance, and umbelliferone, which is characteristic of members of the carrot family. Under certain conditions, coumarin is converted by fungi to the toxic substance dicoumarol. If ingested this causes haemorrhaging and other symptoms of bleeding disorder, by disrupting vitamin K metabolism and preventing the formation of prothrombin and certain other clotting factors by the liver. Dicoumarol and another coumarin derivative, warfarin, are widely used as rodenticides.