CBD and warfarin: why can INR rise?

Medicines and the importance of the interaction

From peer-reviewed publications and drug labels: the warning. The same warning also appears in the interaction checker.

How does the interaction happen?

Warfarin is cleared from the body almost entirely by being broken down in the liver. The more potent half (S-warfarin) is broken down mostly by the CYP2C9 enzyme, and the other half (R-warfarin) by the CYP1A2 and CYP3A4 enzymes. A substance that slows these enzymes can make warfarin stay longer in the blood and cause INR to rise. (Warfarin prescribing information §7.2 and §12.3, PMID 29387536)

CBD is also broken down in the liver by the same enzyme family, mainly CYP3A4 and CYP2C19. In laboratory studies, CBD slowed some of these enzymes. The European product information states that CBD may inhibit CYP2C19 and that a dose reduction should be considered for medicines with a narrow safe range. (EMA Epidyolex prescribing information 4.5 and 5.2, PMID 29387536)

On the other hand, in drug-level studies with approved CBD in healthy volunteers (7.5 mg/kg twice a day), the blood levels of tolbutamide, which is broken down by CYP2C9, and midazolam, which is broken down by CYP3A4, did not change. So the cause of the INR rise seen in the cases is not known for certain. In the reported cases, INR rose as the CBD dose was increased. (FDA Epidiolex label §12.3 (05/2026), EMA Epidyolex prescribing information 4.5 and 5.2, PMID 29387536)

What do studies and official drug information say?

Each finding shows the type of evidence it rests on. A case report describes a single patient; it does not prove cause and effect.

1. Case report

   INR rose with approved CBD (2018)

   A 44-year-old patient with epilepsy who took warfarin because of a mechanical heart valve was started on approved CBD through a program. As the CBD dose was increased every two weeks, INR rose disproportionately (highest 6.86); the warfarin dose was lowered by about 30%. The patient had no bleeding.

   Source: PMID 29387536, PMID 33016308

2. Case report

   In a second case, the dose was lowered by 20% (2020)

   In a 46-year-old patient taking warfarin for a blood clot in a leg vein, INR was closely monitored while approved CBD was started for epilepsy. To keep INR in the target range, the warfarin dose had to be lowered by about 20%.

   Source: PMID 33016308

3. Systematic review

   All cases together: very limited data (2023)

   A systematic review looked at 7 cases of people who took a cannabis product (THC or CBD) while on warfarin: INR rose in 6, 1 had bleeding, and in 4 the weekly warfarin dose was lowered by 22–31%. CBD doses were above 600 mg a day or unknown. The authors rated the quality of the data as very low and said a controlled interaction study is needed. No data showing an interaction with blood thinners other than warfarin were found.

   Source: PMID 37740600

4. Case report

   No clear change at a low dose (2022)

   In an 85-year-old patient taking warfarin, no large swings in INR were seen during a year of using an oral product containing about 5 mg of CBD and 0.3 mg of THC a day. The authors note that the interaction may depend on the dose and on how the product is taken. No general conclusion can be drawn from a single case.

   Source: PMID 35012687

5. Case report

   INR also rose with THC-containing cannabis (2019)

   In a 27-year-old man taking warfarin because of a mechanical heart valve, INR rose to 4.6 after cannabis use. The review in the same publication reports that, in the laboratory, THC slowed the breakdown of warfarin by CYP2C9.

   Source: PMID 30326170

6. Official drug information

   The prescribing information for approved CBD

   Warfarin is not named in the US (Epidiolex, 05/2026) or European (Epidyolex) prescribing information. The level of tolbutamide, which is broken down by CYP2C9, was not affected by CBD; the European product information says that no dose adjustment is needed for medicines broken down by CYP2C9, but that a dose reduction should be considered for medicines broken down by CYP2C19 or with a narrow safe range.

   Source: FDA Epidiolex label §7.2 and §12.3 (05/2026), EMA Epidyolex prescribing information 4.5

7. Official drug information

   Warfarin's label

   The prescribing information for warfarin asks for INR to be measured more often when starting, stopping or changing the dose of another medicine, including herbal products. Patients are advised not to start or stop any other medicine, including herbal products, without talking to their doctor.

   Source: Warfarin prescribing information

What is monitored?

• INR: it is measured more often when CBD is started, when the CBD dose changes and when CBD is stopped. Your doctor decides how often it is measured. (Warfarin prescribing information §5.1, PMID 29387536, PMID 33016308)
• Signs of bleeding: pain, swelling or discomfort; prolonged bleeding from cuts; heavier menstrual bleeding or vaginal bleeding; nosebleeds; bleeding gums when brushing your teeth; unusual bleeding or bruising; red or dark brown urine; red or tar-black stools; headache, dizziness or weakness. If you notice any such sign, tell your doctor right away. (Warfarin prescribing information)
• What is in the product: INR rises have also been reported with THC-containing products. Check the amount of CBD and THC in your product on its label and certificate of analysis. (PMID 30326170, PMID 37740600) How to read a label and certificate of analysis

Frequently asked questions

References

Last reviewed: October 23, 2026. On this date, each source was opened and checked against the related sentence.