Acetylsalicylic acid × Warfarin

Aspirin irreversibly blocks platelet cyclooxygenase and reduces aggregation. Warfarin blocks vitamin K-dependent clotting factor synthesis. Two different antithrombotic mechanisms add up. Aspirin also injures the gastric mucosa, raising bleeding risk independently of coagulation parameters.

Gum bleeding, epistaxis, bruising without trauma, blood in urine or stool, menorrhagia. Severe cases include gastrointestinal or intracranial haemorrhage. Risk rises in patients over 65 and with prior peptic ulcer disease.

High doses of aspirin and other salicylates are contraindicated with warfarin (warfarin SmPC). Analgesic doses of aspirin are not recommended with warfarin and are used only when there is no alternative (aspirin 500 mg SmPC). Aspirin is added to warfarin deliberately, for specific indications: the US warfarin label (FDA) allows it with prosthetic heart valves depending on valve type and position, and for high-risk patients after myocardial infarction lists warfarin plus aspirin up to 100 mg a day. With this combination, monitor the INR carefully (aspirin 100 mg SmPC) and watch for signs of bleeding (US warfarin label). Tell the patient not to start or stop aspirin or other salicylates, including over-the-counter products, without the doctor's advice (US warfarin label).