Therapy with azathioprine or 6-mercaptopurine (6-MP) may interfere with the action of warfarin.

Patient selection: concurrent therapy with azathioprine and warfarin


Clinical features:

(1) evidence of warfarin resistance

(1a) thrombosis despite warfarin therapy

(1b) higher-than-expected dose of warfarin required (2 or more times expected dose)

(2) discontinuation of azathioprine therapy associated with excessive anticoagulation and the need to reduce the warfarin dose

(3) no other explanation for the findings such as noncompliance or vitamin K intake


Similar findings are seen with 6-mercaptopurine.


The mechanism for the warfarin resistance is unclear.

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