- Patients must not be on chronic coumadin therapy for prior medical problems (e.g. cardiac valve prophylaxis); this is due to a presumed interaction with coumadin and ZARNESTRA leading to a significant increase in INR; patients who develop or have recently developed a deep venous thrombosis or pulmonary embolism who are on or will take coumadin will be allowed to participate; however, the investigator should be prepared to monitor patients INR closely