What happens when you take cbd with warfarin?
Warfarin is a narrow-therapeutic-index blood thinner: small changes in its blood level can swing you from under-protected (clot risk) to over-anticoagulated (bleeding risk). Its more potent S-enantiomer is cleared mainly by the liver enzyme CYP2C9. Cannabidiol (CBD) inhibits CYP2C9 (and CYP3A4) at the kinds of exposures people reach with oral CBD, and that is the heart of this interaction.
- CBD blocks the clearing enzyme. CBD inhibits CYP2C9, the enzyme that breaks down warfarin's most active form.
- Warfarin builds up. With the enzyme partly blocked, your liver clears warfarin more slowly, so the amount circulating in your blood rises.
- The INR drifts upward. As warfarin accumulates, your INR climbs, often within days of starting or increasing CBD.
- Bleeding risk increases. A higher-than-intended INR means your blood clots less readily, raising the chance of bruising and bleeding.
A case report from the University of Alabama at Birmingham described a patient on stable warfarin whose INR rose repeatedly as their CBD was increased; the team had to reduce the warfarin dose to keep the INR in a safe range. A systematic review of anticoagulant interactions with cannabinoids reached the same direction of effect.
Why is this important?
An elevated INR is not just a number on a lab report. Bleeding from over-anticoagulation can be subtle (excessive bruising, gum bleeding, prolonged bleeding from small cuts) or serious (gastrointestinal or intracranial bleeding). Older adults and people with high blood pressure, recent surgery, a history of falls, gastritis, or kidney disease are especially vulnerable if the INR drifts up.
Because CBD is sold over the counter as oils, gummies, tinctures, vape liquids, and topicals, many people start it without telling their anticoagulation clinic. Routine INR checks may be weeks apart, which can be long enough for a meaningful shift to go undetected between scheduled draws. The effect is also unpredictable: CBD bioavailability varies widely between products and routes, so two people using what the label calls the same amount can reach very different blood levels, and full-spectrum products that also contain THC add to the enzyme inhibition.
What should you do?
If you are on warfarin and considering CBD, the safest step is to talk to the prescriber or anticoagulation clinic before the first dose. There is no CBD amount established as safe with warfarin, so for many people the plan is careful monitoring rather than guesswork.
Before you change anything:
- Tell your anticoagulation clinic before starting, stopping, or changing any CBD product, including topicals used over large areas of skin.
- Ask what INR monitoring schedule they want, and do not begin until that plan is in place.
Every day while you use it:
- Keep the same product, brand, and route. Switching from oil to gummies, or changing brands, can change how much CBD you absorb and effectively resets your monitoring.
- Watch for bleeding signs: easy bruising, nosebleeds that will not stop, pink or red urine, black tarry stools, coughing up blood, an unusual headache, or vision changes. Seek urgent care for any of these.
- Avoid pairing CBD with other agents that affect the same enzyme (for example certain antifungals, amiodarone, or high-dose fish oil) without specific advice.
After any change:
- Get the INR check your clinic recommends soon after starting or changing CBD, then continue follow-up checks until your readings are stable in range.
- If your INR stays high, your clinic may adjust the warfarin dose, reduce or stop the CBD, or consider an alternative anticoagulant. That is a prescriber decision, not a self-adjustment.
Which specific products are affected?
This is a class effect of cannabidiol, regardless of source. Prescription CBD oral solution (Epidiolex) at anti-seizure doses produces the strongest enzyme inhibition and the most documented INR increases, but consumer products have also caused measurable shifts in case reports. Treat the following as interacting:
- CBD oil tinctures, softgels, and capsules
- CBD gummies and CBD-infused drinks
- Sublingual CBD sprays
- CBD vape cartridges
- Full-spectrum hemp products that also contain delta-9-THC or delta-8-THC (added enzyme burden)
Topical CBD applied to a small joint area has little systemic absorption and is generally lower risk, but patches and balms used over large skin surfaces can produce measurable blood levels and should still be disclosed. Warfarin sold as Coumadin or Jantoven is affected identically.
The science behind it
The clearest evidence is the Grayson case report (Epilepsy Behav Case Rep, 2017; PMID 29387536), which documented a rising INR in a patient as CBD was added to stable warfarin, resolving only after the warfarin dose was reduced. A 2023 systematic review by Smythe and colleagues (Pharmacotherapy) examined anticoagulant interactions with cannabinoids and found consistent reports of INR elevation when CBD or THC products were combined with warfarin. A case report and literature review by Damkier and colleagues (Basic & Clinical Pharmacology & Toxicology, 2019) partly supports the same direction, while noting that the evidence base is largely individual cases rather than controlled trials. Taken together, the human data are case-level rather than large randomized studies, but they consistently point one way: CBD can raise warfarin exposure and INR through CYP2C9 inhibition.
Frequently Asked Questions
Is a little bit of CBD safe with warfarin?
There is no amount established as safe. Even consumer-level CBD has been linked to INR shifts in case reports, and the response varies from person to person. Anything you take should be discussed with your anticoagulation clinic first.
Does the type of CBD product matter?
Yes. Oral and sublingual products (tinctures, capsules, gummies, sprays) reach the bloodstream and can affect warfarin. Full-spectrum products that also contain THC add to the enzyme inhibition. Topicals on small areas absorb little, but large-area balms and patches can still raise blood levels.
How quickly could my INR change?
Case reports describe the INR drifting up within days of starting or increasing CBD, which is why an early INR check after any change is sensible.
Can I just stop the CBD if I notice bleeding?
Stop using it and contact your clinic, but do not assume the problem resolves instantly. Seek urgent care for active bleeding such as blood in urine or stool, black tarry stools, a severe headache, or bleeding that will not stop.
Would switching to a different blood thinner avoid this?
Direct oral anticoagulants are an option some clinics consider, but they have their own potential CBD interactions. Any switch is a prescriber decision, not something to do on your own.
Should I tell my clinic about CBD even if it is over-the-counter?
Yes. Treat any CBD product, prescription or not, as a medication change and disclose it. Your clinic cannot monitor for a risk it does not know about.
Key takeaways
- CBD can slow warfarin breakdown by inhibiting CYP2C9, raising warfarin levels and the INR and increasing bleeding risk.
- The size of the shift is unpredictable and varies by person, product, and route.
- Tell your anticoagulation clinic before starting, stopping, or changing any CBD product.
- Keep the product and route constant, and get the INR checks your clinic recommends after any change.
- Seek urgent care for unusual bruising, nosebleeds that will not stop, blood in urine or stool, black tarry stools, severe headache, or vision changes.
- The evidence is mostly case reports and a systematic review, but it consistently points to a real, clinically meaningful interaction.