Warfarin Drug Interactions Every Pharmacy Tech Must Know

Why Warfarin Interactions Are High-Yield for the PTCE

Warfarin (Coumadin) is one of the most interaction-prone drugs in the pharmacy. It is metabolized by cytochrome P450 enzymes, primarily CYP2C9 and CYP3A4, meaning that anything affecting those enzymes changes warfarin levels. The PTCE frequently tests your ability to identify drugs and foods that increase or decrease warfarin's effect.

Drugs That Increase Warfarin Effect (Raise INR)

These medications raise the risk of bleeding when combined with warfarin:

• Amiodarone - inhibits CYP2C9, one of the most significant interactions
• Fluconazole and metronidazole - antifungal and antibiotic that inhibit warfarin metabolism
• Sulfamethoxazole-trimethoprim (Bactrim) - commonly prescribed antibiotic that potentiates warfarin
• NSAIDs (ibuprofen, naproxen) - increase bleeding risk through antiplatelet effects and GI irritation
• Omeprazole - inhibits CYP2C19, which can modestly raise warfarin levels

Drugs That Decrease Warfarin Effect (Lower INR)

These medications make warfarin less effective, raising clot risk:

• Rifampin - a powerful CYP inducer that dramatically reduces warfarin levels
• Carbamazepine and phenytoin - anticonvulsants that induce hepatic enzymes
• St. John's Wort - an herbal supplement and potent CYP3A4 inducer

Vitamin K and Food Interactions

Vitamin K directly opposes warfarin's mechanism. Patients should maintain consistent intake of vitamin K-rich foods like spinach, kale, broccoli, and Brussels sprouts. The key message is consistency, not avoidance.

Alcohol and Warfarin

Chronic heavy drinking induces CYP enzymes, potentially lowering warfarin effect. Acute binge drinking inhibits metabolism and raises bleeding risk. Moderate, consistent alcohol intake is less problematic than erratic consumption.

When you encounter warfarin interaction questions on the PTCE practice exams, remember to think about whether the interacting drug is an enzyme inhibitor or inducer. That framework helps you predict whether the INR will go up or down.