Pharmacodynamic and pharmacokinetic drug interactions with coumarin anticoagulants

Many drugs alter the anticoagulant effect of oral coumarins. It is essential that physicians be aware of interactions leading to increased or decreased anticoagulation or to erratic control. Interacting drugs can be classified as influencing the pharmacodynamics (pharmacological actions) or the pharmacokinetics (absorption, biotransformation, binding to plasma proteins, excretion) of oral coumarin anticoagulants. Significant pharmacodynamic interactions include those with vitamin K, salicylates, oestrogens, anabolic steroids, phenylbutazone and other anticoagulants. Significant pharmacokinetic interactions include those with cholestyramine, barbiturates, phenylbutazone, rifampicin and chloramphenicol. When prescribing coumarins, the physician should be fully aware of a patient's other drug therapy and the patient should be cautioned against adding new drugs without consultation. If changes in drug therapy are necessary, close monitoring of prothrombin time after the change is essential.