Vitamin K antagonist (oral anticoagulant)
Warfarin drug interactions: a practical overview
Drug interactions are the single biggest cause of preventable medication problems. Warfarin (Warfarin) interacts to varying degrees with several classes of medication and with a smaller list of foods. This page summarises the practically important ones at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg, framed for a real-world prescription review rather than an exhaustive PDF list.
High-priority interactions for Warfarin
For Warfarin, the most clinically relevant interactions are typically with strong CYP3A4 inhibitors and inducers, with cardiovascular medications (notably nitrates for several Vitamin K antagonist (oral anticoagulant) agents), with central nervous system depressants, and with medications affecting blood pressure or heart rate. Warfarin inhibits vitamin K epoxide reductase complex 1 (VKORC1), the enzyme responsible for regenerating reduced vitamin K, a cofactor for the gamma-carboxylation of clotting factors II, VII, IX and…
Working with the pharmacist
A pharmacist review of all current medications is the practical safeguard against unintended interactions with Warfarin. According to the prescribing information for Warfarin, the full medication list — prescription, OTC, supplements and recreational substances — should be reviewed before starting and at every dose change at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg.
Frequently asked questions
What's the most important Warfarin interaction to know? ▾
For most Vitamin K antagonist (oral anticoagulant) medications, the highest-priority interaction is with nitrate medications used for chest pain — this combination is often a hard contraindication. After that, strong CYP3A4 inhibitors (some antifungals, macrolides) are the next concern at routine 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg doses.
Do I need to tell the pharmacist about supplements? ▾
Yes. Supplements and herbal products can interact with Warfarin in ways that prescription drug-drug interaction databases miss. The pharmacist needs the complete picture — including supplements like St John's Wort, grapefruit-containing products and high-dose vitamins — to flag risks at 1mg, 2mg, 2.5mg, 3mg, 4mg, 5mg.
Products containing Warfarin
More on Warfarin
- With alcoholWarfarin and alcohol — is it safe to drink?
- With foodShould Warfarin be taken with food?
- Side effectsWarfarin side effects: common, rare and warning signs
- Dosage guideWarfarin dosage guide: how much to take and when
- OnsetHow fast does Warfarin start working?
- DurationHow long does Warfarin last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.