Hylek et al identified a number of factors associated with excessive warfarin effect. Failure to adjust the warfarin dose when these are present can result in significant bleeding. The authors are from Massachusetts General Hospital and Harvard Medical School.

Risk factors for excessive warfarin anticoagulation:

(1) high dose acetaminophen therapy (especially when >= 1,300 mg per day)

(2) starting a drug that potentiates warfarin's anticoagulant effect

(3) diarrhea

(4) advanced malignancy

(5) taking more warfarin than prescribed


Factors associated with a decreased risk for excessive anticoagulation (but higher risk of insufficient anticoagulation):

(1) increased vitamin K intake

(2) regular, moderate alcohol consumption (up to 2 drinks per day)


If there is a change in any of these factors from normal then it may be necessary to monitor the INR more closely and/or adjust the warfarin dose.


Ways to reduce the risk of excessive or inadequate warfarin anticoagulation:

(1) avoid taking large amounts of acetaminophen except under doctor's instructions

(2) keep vitamin K intake stable

(3) reduce the warfarin dose if significant diarrhea occurs

(4) consider the impact on warfarin  when starting or stopping a drug

(5) carefully adhere to the prescribed dose

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