Aspirin is contraindicated in a patient who is on treatment with:

Aspirin can displace warfarin from protein binding sites elevating the free drug level, leading to prolongation of both bleeding time and prothrombin time. So it should not be used in patients on anticoagulation therapy due to increased risk of bleeding. Other drugs which were found to increase the level of warfarin are: Azapropazone Phenylbutazone Naproxen Mefenamic acid Ibuprofen Ref: Enzyme- and Transpoer-Based Drug-Drug Interactions: Progress and Future Challenges edited by K. Sandy Pang, page 656.