The purpose of this study is to determine if the use of herbal products interferes with normal anticoagulation, leading to either excessive thinning of the blood and bleeding problems or inadequate thinning of the blood and clotting problems. Many patients who are on long term anticoagulation, or blood thinning, for a variety of medical problems, also take herbal products. It is not yet known whether use of herbal products interferes with this anticoagulation, and puts patients at risk for bleeding or clotting. This study will carefully monitor patients who are taking herbs and anticoagulants to determine if their laboratory tests show signs of being affected by the herbs. The study will ask all patients about their herbal product use, so all reported herbs will be included and monitored.

Inclusion criteria:

• Taking longer-term (>6 months) anticoagulation
• Enrolled in one of Kaiser Permanente Northern California anticoagulation clinics