Basic Neuroscience · Neuropharmacology
Warfarin Interactions
Drugs and foods that change the INR in patients on warfarin. Severity reflects the strength of the interaction; monitor INR closely whenever any is started or stopped.
Drugs that increase INR (↑ bleeding risk)
| Category | Drugs |
|---|---|
| Severe interaction | |
| Antimicrobials | Sulfamethoxazole/TMP (Bactrim), Metronidazole, antifungal "-azoles" (fluconazole, voriconazole, ketoconazole, miconazole) |
| Other | Amiodarone, Acetaminophen (high-dose/sustained), Tamoxifen, Fluorouracil, Capecitabine |
| Moderate interaction | |
| Antimicrobials | Azithromycin, Ciprofloxacin, Moxifloxacin, Levofloxacin, Clarithromycin, Erythromycin, Doxycycline, Isoniazid |
| Gastric | Omeprazole, Lansoprazole, Cimetidine |
| Cholesterol | Fenofibrate, Gemfibrozil, Statins (esp. fluvastatin, lovastatin, simvastatin) |
| Nervous system | Quetiapine, SSRIs/SNRIs (paroxetine, fluoxetine, sertraline, citalopram, duloxetine, venlafaxine) |
| Other | Allopurinol, Levothyroxine, Fish oil (high-dose), Cranberry (large amounts) |
Drugs that decrease INR (↑ clotting risk)
| Category | Drugs |
|---|---|
| Severe interaction | |
| Nervous system | Barbiturates, Phenobarbital, Carbamazepine, Primidone, Phenytoin (chronic; early may ↑ INR), St. John's Wort |
| Anti-TB | Rifampin |
| Moderate interaction | |
| Antimicrobials | Dicloxacillin, Nafcillin, Griseofulvin |
| Gastric | Sucralfate, Cholestyramine, Colestipol |
| Cardiac | Bosentan |
| Endocrine | Estrogens / oral contraceptives, Raloxifene, Propylthiouracil, Methimazole, Spironolactone |
| Other | Azathioprine, Sulfasalazine, Mesalamine, Ribavirin, Vitamin K, Coenzyme Q10, Ginseng, Green tea (large amounts) |
Dietary modification
Vitamin-K–rich foods (decrease INR): no need to avoid them, but be consistent with the daily amount to avoid INR swings — kale, spinach, Brussels sprouts, parsley, collard/mustard greens, chard, cabbage, endive, turnip greens, green tea, canola & soybean oil.
Foods that increase INR: alcohol and grapefruit — avoid, or keep to small, consistent amounts.
Note: antiplatelets (aspirin, clopidogrel), DOACs, and NSAIDs raise bleeding risk without necessarily raising the INR — still avoid/co-manage carefully. Strongest INR-raisers act via CYP2C9 inhibition (plus vitamin-K / clotting-factor effects): azoles, TMP-SMX, metronidazole, amiodarone, fluconazole.
Neuro.Wiki by Ahmed Koriesh, MD — Educational only, not a substitute for clinical judgement