To my knowlege, there have been no published reports of CBD causing any clinically significant change in bleeding time, even in anticoagulated patients, despite the theoretical possibility it would compete for metabolism with the Coumadin. This would, in theory, lead to a possible building up of Coumadin level and cause increased anticoagulation. I have never heard of this actually happening. If I were on Coumadin and taking CBD I would just make sure I had my INR checked initially, then faithfully, and if any significant change is noted I would adjust dosages accordingly. It would be best to be able to review this matter with your prescribing physician, but I realize that in the real world that kind of cooperation is still somewhat rare.
The cytochrome p450 system accounts for 70 to 80 percent of the enzymes involved in drug clearance, making it the most important system for drug metabolism. Both THC and CBD are competitive inhibitors of three different enzymes in the cytochrome p450 system, two of which are involved in the metabolism of coumadin. This means that CBD and THC are competing for the same receptors as is coumadin. So when coumadin is taken concurrently with CBD and/or THC, coumadin levels could possibly rise. When coumadin levels rise excessively, blood clotting may be compromised leading to signs ranging from frequent bloody noses all the way on up to a potentially fatal event, such as a hemorrhagic stroke or heart attack.
A case report published in 2009 described a patient who was on coumadin therapy for eleven years. He was admitted to the hospital for a severe gastrointestinal bleed when he confessed to consuming much more cannabis than was typical for him; his INR was 10.41 (therapeutic range is usually between 2 and 3); after inpatient treatment, his INR normalized; then, fifteen days after being discharged, he was readmitted with an INR of 11.55; after he quit using cannabis, his INR remained between 1.08 and 4.40 and he suffered no further complications.
Another case report suggested that extremely high doses of CBD can lead to markedly elevated coumadin levels. Enrolled in a clinical trial evaluating CBD as a potential treatment for epilepsy, the test subject was given extremely high doses of CBD. He was first given doses averaging 265mg, which resulted in a slightly elevated coumadin levels; at doses averaging 528mg, the coumadin levels were markedly elevated; after reducing the coumadin dose, INR results normalized. Relevantly, very few patients, if any, would ever take as much CBD.
However,as previously mentioned, a highly elevated coumadin level can lead to serious consequences. And because the effects of cannabis on coumadin levels have yet to be sufficiently studied, as Dr. Elkind advised, you should discuss the matter with your prescribing physician and monitor your INR results with additional vigilance.