4-Hydroxycoumarins are a class of vitamin K antagonist (VKA) anticoagulant drug molecules. Chemically, they are derived from coumarin by adding a hydroxy group at the 4 position to obtain 4-hydroxycoumarin, then adding a large aromatic substituent at the 3-position (the ring-carbon between the hydroxyl and the carbonyl). The large 3-position substituent is required for anticoagulant activity.
The primary mechanism of the 4-hydroxycoumarin drugs is the inhibition of vitamin K epoxide reductase. These compounds are not direct antagonists (in the pharmaceutical sense) of vitamin K, but rather act to deplete reduced vitamin K in tissues. For this reason vitamin K antagonizes their effect, and this has led to the loose terminology of "vitamin K antagonist".