PTCB Certified Pharmacy Technician (PTCE)MedicationsHard
A patient presents a prescription for 'Warfarin 2 mg, take 1 tablet by mouth daily.' The patient also mentions they have started taking St. John's Wort for mood support. What is the most significant concern regarding this combination?
- AIncreased risk of hypoglycemia
- BDecreased effectiveness of warfarin
- CDecreased effectiveness of St. John's Wort
- DIncreased risk of liver toxicity
Show answer & explanationAnswer & explanation
Correct answer: B. Decreased effectiveness of warfarin
St. John's Wort is a known CYP450 enzyme inducer, particularly CYP3A4. Warfarin is metabolized by CYP enzymes. Inducing these enzymes will increase warfarin metabolism, leading to lower warfarin levels and decreased anticoagulant effect, thereby increasing the risk of blood clots.
Why the other options are wrong
- A. Hypoglycemia is not a primary concern with this interaction; warfarin does not typically cause hypoglycemia.
- C. While St. John's Wort's own effectiveness could be affected by other drugs, the critical interaction here is its effect on warfarin.
- D. While some drugs can cause liver toxicity, this specific interaction primarily concerns altered drug metabolism, not direct hepatotoxicity.
Drug-Herbal Interaction (Warfarin & St. John's Wort)
St. John's Wort can significantly reduce the anticoagulant effect of warfarin by inducing cytochrome P450 enzymes, which metabolize warfarin.
- St. John's Wort is a strong CYP enzyme inducer
- Warfarin's metabolism is increased
- Risk of blood clots increases due to subtherapeutic warfarin levels
Memory trick: Watch Out: St. John's Wort Weakens Warfarin.