Coumarin anticoagulants must be strictly monitored because of their narrow therapeutic index and their potential interactions with other drugs. The high probability of interactions can be explained by two pharmacokinetic properties of coumarins: high binding to plasma albumin (99%), being displaced by other drugs with greater affinity to this protein, and metabolism by liver microsomal enzymes (cytochrome P450), which can be induced or inhibited by other compounds (Shinn & Shrewsbury 1985). A case is reported of a clinically relevant drug interaction of phenytoin and acenocoumarol, possibly potentiated by concomitant treatment with paroxetine, leading to a retroperitoneal haematoma.