Two cases of retroperitoneal haematoma caused by interaction between antibiotics and warfarin.

Several commonly prescribed antibiotics are known to interact with warfarin, increasing its anticoagulant effect by different mechanisms. Retroperitoneal bleeding with consequent haematoma is recognised as a complication of over-anticoagulation. Consequences, which are potentially fatal, include hypovolaemic shock and compression of retroperitoneal structures such as the ureter and inferior vena cava.