Warfarin

For an informed discussion with your doctor or pharmacist. This information collection is not a clinically validated interaction checker. Missing data do not establish safety. Do not change or stop prescribed medicines on the basis of this information. The findings apply to the specific conditions studied; laboratory and animal studies cannot be directly extrapolated to humans.

Active substance: Warfarin

Examples of brand names: Coumadin [USA; historical example]

ATC code: B01AA03

Main use (for orientation): Anticoagulation

Documented research summary

Several case reports suggest possible INR increases with CBD and with THC-containing cannabis; there is no controlled quantitative drug–drug interaction effect. One low-dose case showed no substantial INR change. Smoke, extract, tea and purified CBD must not be equated. Additional finding: Yamreudeewong 2009 reported INR values of 10.41/11.55 with increased marijuana smoking and bleeding; the mechanism is hypothetical. Sauers 2026 (PMID42666628), with INR 9→10.2, head trauma and delayed subarachnoid haemorrhage during exposure to ciprofloxacin, alcohol and marijuana, is a multiple-exposure case, not evidence of isolated cannabinoid causation.

Uncertainties and limitations

Case reports are confounded; no safe dose threshold is established. Laboratory inhibition by CBN/Δ8-THC or CBG-rich extract does not establish a clinical effect.

Separately recorded findings

3 separately recorded findings. Each statement applies to its documented study context.

CBD – clinical signals

Finding ID: B0037

Type of result: Positive finding / indication

Observation: INR increased from 2.22 before CBD to 2.96 at 5 mg/kg/day and 6.86 at 10 mg/kg/day; warfarin remained unchanged at 7.5 mg/day up to that point. Later, warfarin was reduced to an average of 5.36 mg/day (approximately 30% below the initial dose).

Mechanism: CYP inhibition suspected

Study type: Case report

Context studied: One 44-year-old man with a mechanical mitral valve and post-stroke epilepsy; warfarin had previously been stable for at least six months (INR 2.0–2.6). Pharmaceutical CBD (Epidiolex), starting at 5 mg/kg/day in two doses, with planned increases of 5 mg/kg/day every two weeks. INR 6.86 at visit 3 on 10 mg/kg/day.

Clinical interpretation: Potentially high

Limitations and uncertainties: Single case with no reported bleeding; no measured warfarin pharmacokinetics and no general dose rule. Original table: visit 3 is listed as day 28 with the date 23 November 2015, although only 25 calendar days had elapsed since the baseline on 29 October 2015; this timing discrepancy was not independently corrected.

Original sources for this finding: An interaction between warfarin and cannabidiol, a case report [CL13]

CBD – clinical signals

Finding ID: B0038

Type of result: Positive finding / indication

Observation: Approximately 20% lower warfarin requirement after starting CBD.

Mechanism: Mechanism not established

Study type: Case report

Context studied: n=1; pharmaceutical CBD solution.

Clinical interpretation: Potentially high

Limitations and uncertainties: Observation during INR monitoring.

Original sources for this finding: Warfarin dose adjustment required after cannabidiol initiation and titration [CL14]

CBD + Δ9-THC (oromucosal oil) – clinical signals

Finding ID: B0039

Type of result: No / minor effect under the conditions studied

Observation: No substantial INR fluctuations.

Mechanism: PK/PD observed clinically

Study type: Case report: no signal detected

Context studied: n=1; daily CBD 5.3 mg / THC 0.3 mg plus an as-needed product.

Clinical interpretation: No signal in this individual case

Limitations and uncertainties: No safety threshold can be inferred.

Original sources for this finding: Case report: Medical cannabis—warfarin drug-drug interaction [CL15]

References and access status

[CA_L_D0001] WARFARIN SODIUM (WARFARIN) TABLET [A-S MEDICATION SOLUTIONS]

Original source / bibliographic reference

Authors: A-S Medication Solutions

Year: 2026

Source type: Current published human-use SPL / product information

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

[CL13] An interaction between warfarin and cannabidiol, a case report

Original source / bibliographic reference

Authors: Grayson L; Vines B; Nichol K; Szaflarski JP; UAB CBD Program

Year: 2018

Source type: Case report

DOI: 10.1016/j.ebcr.2017.10.001

PMID: 29387536

Source access – 23 October 2026: Automated direct retrieval was restricted; this does not establish that the link is broken.

Bibliographic attribution or source identity cross-checked.

PubMed – bibliographic reference / abstract

DOI – publisher access (access restrictions may apply)

Europe PMC – full-text access according to the literature database – Listed as freely available HTML full text in the official Europe PMC metadata. Direct retrieval was not fully verified here.

[CL14] Warfarin dose adjustment required after cannabidiol initiation and titration

Original source / bibliographic reference

Authors: Cortopassi J

Year: 2020

Source type: Case report

DOI: 10.1093/ajhp/zxaa268

PMID: 33016308

Source access – 23 October 2026: Direct retrieval could not be fully verified; availability remains unresolved.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CL15] Case report: Medical cannabis—warfarin drug-drug interaction

Original source / bibliographic reference

Authors: Thomas TF; Metaxas ES; Nguyen T; Bennett W; Skiendzielewski KV; Quinn DH; Scaletta AL

Year: 2022

Source type: Case report with no relevant INR signal detected

DOI: 10.1186/s42238-021-00112-x

PMID: 35012687

Source access – 23 October 2026: Automated direct retrieval was restricted; this does not establish that the link is broken.

Bibliographic attribution or source identity cross-checked.

PubMed – bibliographic reference / abstract

DOI – publisher access (access restrictions may apply)

Europe PMC – full-text access according to the literature database – Listed as freely available HTML full text in the official Europe PMC metadata. Direct retrieval was not fully verified here.

[CA_P_39703766] Unveiling the impact of water-boiled cannabis on warfarin: A case report of atrial fibrillation patients after cannabis legalization in Thailand.

Original source / bibliographic reference

Authors: Udomuksorn W, Saowaneepitak N, Dolthammasiri P, Ruanglertboon W, Limsuwanchote S, Wiriyapongsukit S, Tungsukruthai S.

Year: 2024

Source type: Journal Article; Case Reports; case-report

DOI: 10.1016/j.toxrep.2024.101838

PMID: 39703766

Source access – 23 October 2026: Direct retrieval could not be fully verified; availability remains unresolved.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

PubMed Central – full-text access according to the literature database – Archive attribution confirmed by Europe PMC metadata. Direct retrieval was not fully verified here; access may require a browser check.

Europe PMC – full-text access according to the literature database – Listed as freely available HTML full text in the official Europe PMC metadata. Direct retrieval was not fully verified here.

[CA_P_32828704] Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.

Original source / bibliographic reference

Authors: Brown GW, Bellnier TJ, Janda M, Miskowitz K.

Year: 2021

Source type: Journal Article; Case Reports

DOI: 10.1016/j.japh.2020.07.028

PMID: 32828704

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CA_P_30326170] Interaction between warfarin and cannabis.

Original source / bibliographic reference

Authors: Damkier P, Lassen D, Christensen MMH, Madsen KG, Hellfritzsch M, Pottegård A.

Year: 2019

Source type: Review; Journal Article; Case Reports

DOI: 10.1111/bcpt.13152

PMID: 30326170

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CA_P_31319733] Probable Interaction Between Warfarin and Inhaled and Oral Administration of Cannabis.

Original source / bibliographic reference

Authors: Hsu A, Painter NA.

Year: 2020

Source type: Journal Article; Case Reports

DOI: 10.1177/0897190019854958

PMID: 31319733

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CA_P_40829519] Inhibitory effects of Δ8-tetrahydrocannabinol on major hepatic cytochrome P450 enzymes and implications for drug disposition.

Original source / bibliographic reference

Authors: Zhao M, Coates S, Bardhi K, Lazarus P.

Year: 2025

Source type: research-article; Journal Article

DOI: 10.1016/j.dmd.2025.100122

PMID: 40829519

Source access – 23 October 2026: Direct retrieval could not be fully verified; availability remains unresolved.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

PubMed Central – full-text access according to the literature database – Archive attribution confirmed by Europe PMC metadata. Direct retrieval was not fully verified here; access may require a browser check.

Europe PMC – full-text access according to the literature database – Listed as freely available HTML full text in the official Europe PMC metadata. Direct retrieval was not fully verified here.

[CA_P_36814687] Phytochemical Comparison of Medicinal Cannabis Extracts and Study of Their CYP-Mediated Interactions with Coumarinic Oral Anticoagulants.

Original source / bibliographic reference

Authors: Treyer A, Reinhardt JK, Eigenmann DE, Oufir M, Hamburger M.

Year: 2023

Source type: research-article; Journal Article

DOI: 10.1159/000528465

PMID: 36814687

Source access – 23 October 2026: Direct retrieval could not be fully verified; availability remains unresolved.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

PubMed Central – full-text access according to the literature database – Archive attribution confirmed by Europe PMC metadata. Direct retrieval was not fully verified here; access may require a browser check.

Europe PMC – full-text access according to the literature database – Listed as freely available HTML full text in the official Europe PMC metadata. Direct retrieval was not fully verified here.

[CA_P_22166891] Comparison in the in vitro inhibitory effects of major phytocannabinoids and polycyclic aromatic hydrocarbons contained in marijuana smoke on cytochrome P450 2C9 activity.

Original source / bibliographic reference

Authors: Yamaori S, Koeda K, Kushihara M, Hada Y, Yamamoto I, Watanabe K.

Year: 2012

Source type: Comparative Study; Research Support, Non-U.S. Gov't; Journal Article

DOI: 10.2133/dmpk.dmpk-11-rg-107

PMID: 22166891

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CA_P_19531696] Probable interaction between warfarin and marijuana smoking.

Original source / bibliographic reference

Authors: Yamreudeewong W, Wong HK, Brausch LM, Pulley KR.

Year: 2009

Source type: Journal Article; Case Reports

DOI: 10.1345/aph.1m064

PMID: 19531696

Source access – 23 October 2026: Direct retrieval could not be fully verified; availability remains unresolved.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

[CA_P_42666628] Delayed Traumatic Subarachnoid Hemorrhage Following Initially Negative Computed Tomography (CT) Imaging in a Patient With a Mechanical Mitral Valve and Markedly Elevated International Normalized Ratio (INR).

Original source / bibliographic reference

Authors: Sauers KN, Tomlinson PL, Hall TE.

Year: 2026

Source type: Journal Article; Case Reports; case-report

DOI: 10.7759/cureus.113580

PMID: 42666628

Source access – 23 October 2026: Source destination was accessible when checked.

Bibliographic attribution or source identity cross-checked.

DOI – publisher access (access restrictions may apply)

Research as of 23 October 2026. Not all original full texts could be reviewed in full. Source accessibility and the strength of scientific evidence must be assessed separately. An accessible link does not establish medical approval; a bibliographic record with an abstract does not replace the original full text.

Drug search and explanation of the evidence · Guide: cannabinoids and medicines · Author and responsibility