Drug General Information (ID: DDIAVJ30LG)
Drug Name			Warfarin		Drug Info		Mercaptopurine		Drug Info
Drug Type		Small molecule				Small molecule
Therapeutic Class		Anticoagulants				Antineoplastics
Structure

Mechanism of Warfarin-Mercaptopurine Interaction (Severity Level: Moderate)
Antagonize the effect of antithrombotic agents Click to Show/Hide Mechanism Graph
Drug Name		Warfarin				Mercaptopurine
Mechanism		Vitamin K antagonist				Decrease the hypoprothrombinemic effect of vitamin K antagonist
Key Mechanism Factor 1
		Factor Name		Antithrombotic agents
		Factor Description		The beneficial effects of antithrombotic agents are reduced, leading to an increased risk of thromboembolism, stroke and/or myocardial infarction.
Mechanism Description		Antagonize the effect of Warfarin when combined with Mercaptopurine

Recommended Action
Management		Given the potential for interaction and the narrow therapeutic index of oral anticoagulants, patients should be closely monitored during concomitant therapy with azathioprine or mercaptopurine. The INR should be checked frequently and anticoagulant dosage adjusted accordingly, particularly following initiation or discontinuation of antimetabolite therapy in patients who are stabilized on their anticoagulant regimen. Patients should be advised to promptly report any signs of blood clots (e.g. chest pain, shortness of breath, sudden loss of vision, or pain, redness or swelling in an extremity).

References
1	Havrda DE, Rathbun S, Scheid D "A case report of warfarin resistance due to azathioprine and review of the literature." Pharmacotherpy 21 (2001): 355-7. [PMID: 11253860]
2	Rivier G, Khamashta MA, Hughes GR "Warfarin and azathioprine: a drug interaction does exist." Am J Med 95 (1993): 342. [PMID: 8368235]
3	Rotenberg M, Levy Y, Shoenfeld Y, Almog S, Ezra D "Effect of azathioprine on the anticoagulant activity of warfarin." Ann Pharmacother 34 (2000): 120-2. [PMID: 10669196]
4	Singleton JD, Conyers L "Warfarin and azathioprine: an important drug interaction." Am J Med 92 (1992): 217. [PMID: 1543209]
5	Spiers AS, Mibashan RS "Increased warfarin requirement during mercaptopurine therapy: a new drug interaction." Lancet 2 (1974): 221-2. [PMID: 4135641]
6	Walker J, Mendelson H, McClure A, Smith MD "Warfarin and azathioprine: clinically significant drug interaction." J Rheumatol 29 (2002): 398-9. [PMID: 11838865]