Answer
St John's wort increases warfarin metabolism and reduces its anticoagulant effect, which may persist for at least two weeks after stopping the herb, so they must not be used together.
Sources for this answer
- Warfarin Sodium 1mg/1ml Oral SuspensionRosemont Pharmaceuticals Limited · emc · SmPC §4.5 Interaction with other medicinal products · Version 12Document updated
Sources checked