Wfarin Interactions Flashcards
(9 cards)
How is warfarin therapy monitored?
Warfarin is monitored using the INR (International Normalized Ratio).
Target INR is typically 2–3, or 2.5–3.5 for mechanical heart valves.
Which enzyme primarily metabolizes warfarin?
Warfarin is metabolized mainly by CYP2C9, part of the cytochrome P450 system.
What happens to INR when a CYP450 enzyme inducer is added?
Enzyme inducers increase warfarin metabolism, which decreases INR, leading to increased risk of clotting.
Which drugs are common CYP450 inducers that lower INR?
Carbamazepine, Rifampin, Phenytoin (biphasic effect: initial ↑INR, then ↓INR), Phenobarbital, St. John’s Wort.
Mnemonic: CRaPPS
What happens to INR when a CYP450 inhibitor is added?
Enzyme inhibitors decrease warfarin metabolism, which increases INR, raising the risk of bleeding.
Which drugs are common CYP450 inhibitors that raise INR?
Amiodarone, Azole antifungals (e.g. fluconazole), Macrolides (e.g. erythromycin), Cimetidine, Grapefruit juice.
Mnemonic: “AAA RACK”
How does warfarin interact with leafy green vegetables?
Leafy greens are high in vitamin K, which antagonizes warfarin → decreases INR.
Tip: It’s not about avoiding them completely — just maintain consistent intake.
How do antibiotics affect warfarin levels?
Some antibiotics kill gut flora that synthesize vitamin K, leading to ↑ INR and bleeding risk.
What is the reversal agent for warfarin toxicity?
Vitamin K (phytonadione) – for non-urgent reversal. Fresh frozen plasma (FFP) or Prothrombin Complex Concentrate (PCC) – for urgent reversal (e.g., active bleeding).
