Retatrutide and Warfarin: A Completed Study With No Posted Results

The Crucial Answer Is “Studied, Outcome Unpublished”

The legacy version said no formal retatrutide–warfarin interaction study existed. That is incorrect. ClinicalTrials.gov record NCT05445232 describes a multiple-dose Phase 1 study of retatrutide (LY3437943) with an oral cocktail containing midazolam, warfarin, and caffeine.

The study began July 8, 2022, enrolled 32 participants with obesity, and completed February 24, 2023. Its listed warfarin endpoints were area under the concentration–time curve from zero to infinity and maximum observed concentration, sampled through 96 hours. The record says the purpose was to determine retatrutide’s effect on the blood levels of the cocktail drugs.

Yet the registry’s results section is empty. It does not publish numerical warfarin exposure ratios, confidence intervals, INR results, bleeding outcomes, or a dose-adjustment conclusion. “Completed” means participant follow-up ended; it does not mean the answer is publicly available.

Evidence Ledger for the Interaction Question

The evidence does not support either “no interaction” or the legacy page’s “moderate-to-high” severity label. It supports unresolved direct evidence plus an already narrow-therapeutic-index medicine.

Warfarin Monitoring Does Not Need an Invented Retatrutide Rule

Warfarin’s anticoagulant effect varies with medicines, illness, dietary vitamin K, alcohol, adherence, and other factors. Its FDA-approved labeling states:

“Perform regular monitoring of INR in all treated patients.”

The issuer is the U.S. Food and Drug Administration in the Coumadin prescribing information. This eight-word boxed-warning direction applies to warfarin regardless of retatrutide and does not endorse either product (FDA Coumadin label, 2015, boxed warning and section 5.1). The same label calls for more frequent monitoring when drugs are started, stopped, or changed, but frequency remains clinical rather than a universal weekly-for-six-weeks rule.

Because no retatrutide result is public, a warfarin prescriber or anticoagulation service should know about any investigational or nonapproved product exposure. They can choose INR timing based on the person’s stability, indication, bleeding and thrombosis risk, diet, illness, and medication changes. Do not change a warfarin dose from a webpage.

The retatrutide interaction evidence map covers the broader oral-medication question.

Why Gastric Emptying Is Not an INR Result

In a 72-participant study, retatrutide delayed the time to peak of oral acetaminophen, used as a gastric-emptying marker, while total acetaminophen exposure was not affected (Urva et al., DOI 10.1111/dom.15167). The result establishes a motility effect under that protocol.

It does not show that warfarin absorption becomes “erratic,” that INR will rise, or that the effect lasts four to six weeks. Vomiting, reduced food intake, acute illness, and changing vitamin K intake may matter to warfarin management, but each is a clinical pathway—not a measured retatrutide interaction in the available record.

Semaglutide Is Useful Indirect Evidence, Not a Surrogate Answer

Hausner and colleagues gave a single 25 mg warfarin dose before and during steady-state subcutaneous semaglutide in healthy volunteers. Warfarin exposure stayed within the study’s prespecified no-interaction bounds, and no clinically relevant INR-response change was observed (PMID 28349387; DOI 10.1007/s40262-017-0532-6). An oral-semaglutide study similarly found no apparent warfarin exposure effect (PMID 30945118).

Those findings correct the legacy claim that semaglutide provides evidence of clinically meaningful INR fluctuation. But semaglutide and retatrutide are different molecules, and a single-dose healthy-volunteer study is not chronic warfarin care in people with changing illness, diet, and body weight. The studies are reassuring class context, not permission to fill in NCT05445232’s missing results.

A 2026 retrospective cohort of 1,021 warfarin users found a modest 2.1-percentage-point average decrease in time in therapeutic range during six months after GLP-1 receptor agonist initiation, with unchanged mean INR and slightly greater variability (PMID 42115581). That association can inform vigilance, but it cannot isolate retatrutide or prove a pharmacokinetic interaction.

A Practical Warfarin Review

Give the anticoagulation clinician the exact product name, source, dose, date started or changed, and any available packaging or lot information. Also report:

• missed warfarin doses or vomiting after a dose;
• major changes in food intake, especially vitamin K–rich foods;
• diarrhea, dehydration, fever, antibiotics, new supplements, alcohol changes, or acute illness;
• unusual bruising, persistent bleeding, red or dark urine, black stools, coughing or vomiting blood, severe headache, fainting, or new neurologic symptoms;
• leg swelling, chest pain, shortness of breath, or other possible thrombosis symptoms.

Severe bleeding, head injury while anticoagulated, stroke symptoms, chest pain, or major breathing difficulty requires emergency evaluation. Do not wait for the next routine INR.

FDA separately states that retatrutide is not approved, has not been found safe and effective, and cannot be compounded under federal law (FDA current warning). That product-status problem matters because a nontrial vial cannot inherit the identity and purity of the trial material used in NCT05445232.

The retatrutide safety-signal review explains those product-quality boundaries, and the adult monitoring guide keeps investigational-drug follow-up separate from anticoagulation management.

What the Legacy Page Got Wrong

The previous page said no direct study existed, then asserted that delayed gastric emptying caused unpredictable INR shifts, that semaglutide and tirzepatide had shown clinically meaningful fluctuation, and that all users needed weekly INR tests through an invented stabilization period. It also claimed weight-loss changes in warfarin volume of distribution without a supporting retatrutide study.

The corrected page does something more useful: names the direct trial, exposes its missing results, accurately characterizes indirect evidence, and keeps warfarin’s established monitoring rule intact without fabricating a combination protocol.

Medical review of this revision is pending. FDA, ClinicalTrials.gov/NLM, investigators, authors, journals, sponsors, manufacturers, and institutions do not endorse retatrutide products, HealthRX.com, or this page.