warfarin
Brand names: Coumadin, Jantoven
Warfarin is a medicine that helps prevent blood clots. It is used to treat and prevent dangerous clots from forming in your body.
Drug Pricing (NADAC)
Generic Price
$0.09/unit
Generic Available
Yes (5 manufacturers)
Pricing data from NADAC (CMS), effective December 18, 2024. Compare all drug costs →
What it does
Warfarin is used to prevent and treat blood clots in your veins and lungs.
Common side effects
Bleeding from any tissue or organ
Key warnings
Warfarin can cause major or fatal bleeding.
How It Works
Warfarin works by blocking your body's use of vitamin K. Vitamin K is needed to make blood clotting factors. By blocking vitamin K, warfarin makes your blood less likely to clot.
How to Take It
Take warfarin exactly as your doctor tells you. Your doctor will adjust your dose based on regular blood tests (INR). The goal is to keep your INR in the right range. Follow your doctor's instructions carefully and do not change the dose yourself.
Pregnancy & Breastfeeding
Warfarin can harm your unborn baby, especially during the first three months of pregnancy. Do not take warfarin if you are pregnant, unless you have a mechanical heart valve and your doctor says the benefits outweigh the risks. Talk to your doctor if you are breastfeeding, and watch your baby for bruising or bleeding.
Missed Dose
If you miss a dose of warfarin, take it as soon as you remember, unless it is almost time for your next dose. Do not take two doses at once.
Storage
Store warfarin at room temperature (68° to 77°F) in a tightly closed container, away from light.
Side Effects (from patient reports)
Based on 62,747 FDA adverse event reports.
FDA Adverse Event Report Analysis
Detailed analysis of 124,873 reports from the FDA Adverse Event Reporting System (FAERS). Reports span 1999–2025.
Total Reports
124,873
Death-Related Reports
15,949
Hospitalization Reports
57,230
Top Indication
Product Used For Unknown Indication
Gender Distribution
Age Distribution
Most Reported Adverse Reactions (FAERS)
| # | Reaction | Reports |
|---|---|---|
| 1 | INTERNATIONAL NORMALISED RATIO INCREASED | 10,276 |
| 2 | DYSPNOEA | 8,408 |
| 3 | DRUG INTERACTION | 6,288 |
| 4 | FATIGUE | 6,143 |
| 5 | NAUSEA | 5,921 |
| 6 | DIARRHOEA | 5,909 |
| 7 | DIZZINESS | 5,166 |
| 8 | DRUG INEFFECTIVE | 5,113 |
| 9 | FALL | 4,809 |
| 10 | DEATH | 4,718 |
| 11 | HEADACHE | 4,651 |
| 12 | ASTHENIA | 4,369 |
| 13 | PNEUMONIA | 4,097 |
| 14 | HAEMORRHAGE | 4,036 |
| 15 | ANAEMIA | 3,928 |
Reactions in Death Reports
Reactions in Hospitalization Reports
Source: FDA FAERS (Adverse Event Reporting System) FDA FAERS (Adverse Event Reporting System) Reports are voluntary and do not establish causation
Serious Warnings
Warfarin can cause major or fatal bleeding. You must have your blood tested regularly (INR) while taking warfarin. Many things, like other medicines and diet changes, can affect your INR. Tell your doctor about any bleeding and follow their instructions to prevent bleeding.
Known Drug Interactions
NSAIDs, Aspirin, Warfarin): May potentiate the risk of bleeding ( 7.4 ) Drugs Tightly Bound to Plasma Proteins: May cause a shift in plasma concentrations ( 7.6 , 7.7 ) Olanzapine: When used in combination with fluoxetine, also refer to the Drug Interactions section of the package insert for Symbyax ( 7.7 ) Drugs that Prolong the QT Interval: Do not use fluoxetine with thioridazine or pimozide. 7.4 Drugs that Interfere with Hemostasis (e.g., NSAIDs, Aspirin, Warfarin) Serotonin release by platelets plays an important role in hemostasis. Altered anticoagulant effects, including increased ble...
Mechanism: Fluoxetine can change how your blood thinner works and affect how your blood clots, making it more likely for you to bleed.
What to do: Your doctor will need to monitor your blood clotting levels very closely and may need to adjust your warfarin dose.
( 7.1 ) Warfarin: Do not use in patients taking tamoxifen for DCIS and for reduction in breast cancer incidence in women at high risk. Warfarin A marked increase in anticoagulant effect may occur when tamoxifen is used in combination with warfarin. Closely monitor coagulation indices in patients who are taking tamoxifen for either the treatment of metastatic breast cancer or as adjuvant therapy who require concomitant use of warfarin [see Contraindications (4) ] .
Mechanism: Tamoxifen increases the blood-thinning effects of warfarin, which can lead to a much higher risk of dangerous bleeding.
What to do: Avoid using these together if you are taking tamoxifen for cancer prevention, or have your doctor check your blood clotting levels very often.
Table 3: Drugs that Can Increase the Risk of Bleeding Drug Class Specific Drugs Anticoagulants argatroban, dabigatran, bivalirudin, desirudin, heparin, lepirudin Antiplatelet Agents aspirin, cilostazol, clopidogrel, dipyridamole, prasugrel, ticlopidine Non-steroidal Anti-Inflammatory Agents celecoxib, diclofenac, diflunisal, fenoprofen, ibuprofen, indomethacin, ketoprofen, ketorolac, mefenamic acid, naproxen, oxaprozin, piroxicam, sulindac Serotonin Reuptake Inhibitors citalopram, desvenlafaxine, duloxetine, escitalopram, fluoxetine, fluvoxamine, milnacipran, paroxetine, sertraline, venlafa...
Mechanism: Ibuprofen is an anti-inflammatory drug that increases the risk of bleeding when taken with warfarin. Both medications interfere with the body's natural ability to form blood clots.
What to do: Your doctor should monitor you closely for signs of bleeding, such as unusual bruising. Report any dark or bloody stools to your healthcare provider immediately.
Table 3: Drugs that Can Increase the Risk of Bleeding Drug Class Specific Drugs Anticoagulants argatroban, dabigatran, bivalirudin, desirudin, heparin, lepirudin Antiplatelet Agents aspirin, cilostazol, clopidogrel, dipyridamole, prasugrel, ticlopidine Non-steroidal Anti-Inflammatory Agents celecoxib, diclofenac, diflunisal, fenoprofen, ibuprofen, indomethacin, ketoprofen, ketorolac, mefenamic acid, naproxen, oxaprozin, piroxicam, sulindac Serotonin Reuptake Inhibitors citalopram, desvenlafaxine, duloxetine, escitalopram, fluoxetine, fluvoxamine, milnacipran, paroxetine, sertraline, venlafa...
Mechanism: Aspirin is an antiplatelet drug that adds to the blood-thinning effect of warfarin. Taking both together significantly increases your chance of having a serious bleeding event.
What to do: Talk to your doctor before taking these two drugs together. They may need to adjust your dosage or check your blood levels more frequently.
Table 3: Drugs that Can Increase the Risk of Bleeding Drug Class Specific Drugs Anticoagulants argatroban, dabigatran, bivalirudin, desirudin, heparin, lepirudin Antiplatelet Agents aspirin, cilostazol, clopidogrel, dipyridamole, prasugrel, ticlopidine Non-steroidal Anti-Inflammatory Agents celecoxib, diclofenac, diflunisal, fenoprofen, ibuprofen, indomethacin, ketoprofen, ketorolac, mefenamic acid, naproxen, oxaprozin, piroxicam, sulindac Serotonin Reuptake Inhibitors citalopram, desvenlafaxine, duloxetine, escitalopram, fluoxetine, fluvoxamine, milnacipran, paroxetine, sertraline, venlafa...
Mechanism: Diclofenac is an anti-inflammatory medicine that can increase the risk of bleeding when used with warfarin. This combination makes it harder for your blood to clot properly.
What to do: Avoid using these medications together unless your doctor specifically tells you to. If you must take both, watch closely for any signs of bleeding or bruising.
Common Questions
What is INR?
How often will I need blood tests?
What should I do if I start a new medicine?
Can I eat whatever I want while taking warfarin?
What are the signs of bleeding?
What should I do if I have a minor cut?
Can I drink alcohol while taking warfarin?
What if I need surgery or a dental procedure?
Can I take aspirin or ibuprofen for pain?
What should I do if I fall or hit my head?
What are the common side effects of warfarin?
Does warfarin interact with other medications?
What drug class is warfarin?
Is warfarin safe during pregnancy?
Related Medications in Vitamin K Antagonist (Anticoagulant)
Other drugs grouped near warfarin — same-class peers and common alternatives.
apixaban
Eliquis
Apixaban (Eliquis) is a medicine that helps prevent blood clots.
Compare with warfarin →
aspirin
Bayer, Ecotrin
Aspirin is a common medicine used to relieve minor pain.
Compare with warfarin →
cilostazol
Pletal
Cilostazol is a medicine that helps improve walking distance in people with leg pain due to poor circulation.
Compare with warfarin →
clopidogrel
Plavix
Clopidogrel is a drug that helps to prevent blood clots.
Compare with warfarin →
dabigatran
Pradaxa
Dabigatran (Pradaxa) is a drug that helps to prevent blood clots from forming.
Compare with warfarin →
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What the FDA Data Shows for warfarin
The FDA label for warfarin (sold under brand names such as Coumadin, Jantoven) classifies it as a prescription-only medication in the Vitamin K Antagonist (Anticoagulant) class. Warfarin is used to prevent and treat blood clots in your veins and lungs. Official labeling lists 1 commonly reported side effect, including Bleeding from any tissue or organ.
Post-market surveillance from the FDA Adverse Event Reporting System (FAERS) captures real-world experience. For this drug, FAERS contains 62,747 voluntary reports. The database also lists 163 documented drug interactions derived from FDA labeling, with the top-flagged interaction rated major severity. NADAC pricing from CMS shows a generic unit cost of $0.09.
Report counts do not establish causation — a FAERS entry documents a temporal association, not proof that the drug produced the outcome. Widely prescribed medications naturally accumulate more reports than niche therapies, so raw totals must be interpreted alongside total exposure. Shortage status, recall history, and patent information further shape supply and switching decisions. This page summarizes public FDA data for educational purposes only and is not a substitute for professional medical advice — always consult a licensed healthcare provider before starting, stopping, or changing any medication.
Data Sources
Drug labeling: FDA Drug Labels (SPL/DailyMed). Adverse events: FDA Adverse Event Reporting System (FAERS). Pricing: CMS National Average Drug Acquisition Cost (NADAC).
FAERS reports are voluntary and do not establish causation. Drug interactions are derived from FDA labeling and clinical references. Always consult a healthcare professional before making medication decisions.
Last updated: June 17, 2025
Read our methodology — how this data is sourced, computed, and verified.
All federal data sources used on this page
- FDA Orange Book — approved drug products with therapeutic equivalence. accessdata.fda.gov/cder/ob
- FDA DailyMed — NIH-hosted drug labeling for FDA-approved meds. dailymed.nlm.nih.gov
- FDA Adverse Event Reporting System (FAERS) — post-marketing safety surveillance. fda.gov/drugs/faers
- NLM RxNorm — standardized clinical drug nomenclature. nlm.nih.gov/research/umls/rxnorm
- CMS Medicare Part B Drug Average Sales Price Files — federal drug pricing data. cms.gov/medicare/part-b-drugs/asp
- FDA Drug Shortages Database — current and resolved drug shortage tracking. accessdata.fda.gov/drugshortages