What happens when you take fluconazole with warfarin?
Warfarin is one of the most heavily prescribed oral anticoagulants worldwide, used to prevent stroke in atrial fibrillation, treat venous thromboembolism, and protect patients with mechanical heart valves. Its therapeutic window is narrow: too little and clots form, too much and serious bleeding occurs. The drug is metabolized primarily by CYP2C9, with smaller contributions from CYP3A4 and CYP1A2.
Fluconazole, sold most often as Diflucan and as generic fluconazole, is an azole antifungal commonly prescribed for vaginal yeast infections (often a single 150 mg oral dose), oral and esophageal thrush, and systemic candidiasis. It is a potent inhibitor of CYP2C9 and a moderate inhibitor of CYP3A4. When fluconazole is added to chronic warfarin therapy, warfarin clearance falls sharply, plasma concentrations climb, and the anticoagulant effect intensifies. The reported magnitude is large: warfarin effect can increase by 50 to 100 percent or more, depending on fluconazole dose and treatment duration. The onset is fast, often producing a clinically meaningful INR rise within 2 to 3 days of starting fluconazole.
Why is this important?
This is a top-tier interaction in pharmacology textbooks because it has caused documented major hemorrhage and death. Published case reports describe patients on stable warfarin who received even a single 150 mg dose of fluconazole for a yeast infection and developed INRs greater than 10 within days, with gastrointestinal bleeding, intracranial hemorrhage, or fatal outcomes. A 2018 Lancet case report specifically documented bleeding from this combination.
Two features make this interaction especially dangerous. First, the fluconazole prescription is often a one-off, taken outside the warfarin-prescribing clinic's awareness. A patient may get fluconazole from urgent care, an urgent telehealth visit, or a gynecology office, and the warfarin-monitoring clinic may not learn about it until the next scheduled INR check, by which time bleeding may already have started. Second, fluconazole has a long half-life of about 30 hours in adults, so a single 150 mg dose continues to inhibit CYP2C9 for several days. The interaction window extends well past the day the antifungal is taken.
The risk is concentration-dependent on both sides. Higher fluconazole doses (200 to 800 mg per day for systemic infections) produce larger and more sustained CYP2C9 inhibition. Patients who are CYP2C9 poor metabolizers (carriers of certain CYP2C9*2 or *3 variants) start with reduced enzyme capacity and are especially vulnerable.
What should you do?
If you take warfarin and are prescribed fluconazole for any reason, including a single-dose treatment for vaginal yeast, immediately tell the warfarin-managing clinician or anticoagulation pharmacist. Standard practice is to obtain an INR before or on the day of fluconazole initiation, repeat the INR within 2 to 3 days, and then every 2 to 3 days until both the fluconazole course is complete and the INR has returned to baseline.
Most anticoagulation services will preemptively reduce the warfarin dose by 25 to 50 percent at the start of a fluconazole course, depending on dose and duration, and then re-titrate based on INR results. Do not adjust your warfarin dose on your own based on internet guidance: the magnitude of dose change needed varies by individual.
Be alert to signs of overcoagulation during and for at least one week after the fluconazole course: nosebleeds that do not stop quickly, gum bleeding, easy or excessive bruising, blood in urine or stool, black tarry stool, unusual headache, vision changes, or weakness. If any of these occur, seek urgent medical evaluation and have an INR drawn.
Which specific products are affected?
This warning applies to all oral and IV fluconazole formulations (Diflucan tablets, Diflucan oral suspension, generic fluconazole, fluconazole injection) at any dose, including the single 150 mg dose for vaginal candidiasis. Topical antifungals like clotrimazole and miconazole cream produce minimal systemic absorption and have a much smaller interaction risk, though intravaginal miconazole has occasionally been reported to elevate INR and should also be mentioned to the prescriber.
The warfarin side includes brand-name Coumadin and Jantoven and all generic warfarin products. The interaction is class-specific: other azole antifungals, including ketoconazole, itraconazole, voriconazole, and miconazole, also inhibit CYP2C9 to varying degrees and similarly elevate warfarin effect. Terbinafine has a smaller but still notable effect. Topical nystatin and oral nystatin suspensions have minimal absorption and minimal interaction.
The bottom line
Fluconazole plus warfarin is a high-severity interaction that has caused fatal bleeding. Even a single 150 mg dose for yeast infection can push the INR into a dangerous range within days. Always inform the warfarin-managing clinician before starting any azole antifungal, and ensure INR is monitored within 2 to 3 days of fluconazole initiation and throughout the course.