Educational use only. Drug cards are AI-assisted study material for NCLEX preparation.
Mechanism of action
Inhibits hepatic synthesis of vitamin K–dependent clotting factors (II, VII, IX, X) and proteins C and S. Onset is slow (3–5 days), so heparin is typically bridged until the INR is therapeutic. Used for long-term anticoagulation in atrial fibrillation, mechanical valves, DVT/PE, and post-stroke.
Adverse effects
Life-threatening / NCLEX-tested
- Major bleeding — GI bleed, intracranial hemorrhage, retroperitoneal bleed
- Hemorrhagic stroke
- Skin necrosis (rare; occurs early, often on breasts/buttocks/thighs in protein C–deficient patients)
- Purple toe syndrome
- Teratogenic — absolute contraindication in pregnancy
Side effects
Common — what to teach
- Easy bruising
- Minor bleeding (gums, nosebleeds)
- Heavier menstrual flow
- Nausea, anorexia
- Hair loss
Food & drug interactions
Many — warfarin is one of the most interaction-prone drugs in nursing. Antibiotics (especially Bactrim, metronidazole, fluconazole, macrolides), amiodarone, NSAIDs, aspirin, and SSRIs all increase bleeding risk. Vitamin K reduces effect — teach consistent, not zero, intake of green leafy vegetables (spinach, kale, broccoli). Cranberry juice and grapefruit can raise INR. Alcohol effect is dose-dependent — chronic use lowers INR, acute binge raises it.
Nursing implications
Assessment, monitoring, patient teaching
- Monitor INR regularly; therapeutic range is 2.0–3.0 for most indications and 2.5–3.5 for mechanical mitral valves
- Antidote is vitamin K (phytonadione); for serious bleeds use 4-factor PCC (Kcentra) or FFP
- Teach patients consistent vitamin K intake — do not stop greens, but do not binge on them either
- Use a soft toothbrush and electric razor; avoid contact sports
- Report black/tarry stools, blood in urine, severe headache, or unusual bruising immediately
- Wear a medical alert ID; carry a current med list
- Pregnancy test before starting in women of childbearing age — warfarin is teratogenic
When to hold / contraindications
- INR > 4.0 with no bleeding (consider holding 1–2 doses, recheck)
- INR > 5.0 (hold and consider oral vitamin K)
- Active bleeding from any site
- Scheduled invasive procedure or surgery (typically held 5 days prior)
- Confirmed or suspected pregnancy
Memory anchor
"Warfarin works on 1972" — vitamin K factors II, VII, IX, X. INR goal 2–3 (or 2.5–3.5 for mechanical valves). Antidote: vitamin K. Consistent greens, not zero greens.
Practice Warfarin questions
Test your recall on real NCLEX-style pharmacology questions — with full rationales and an AI Coach for the parts you miss.