What this means in practice
Tell the anticoagulation clinic before starting or stopping St John's Wort, so the INR is checked on a tighter schedule through the change rather than at the usual interval. The routine blood test is what makes this interaction manageable rather than silent, which is not true of most of the others on this list.
What people say
Anything that interferes with a blood thinner must make bleeding more likely, so the danger sign to watch for is bruising.
What the evidence supports
The UK medicines regulator's pharmacovigilance review identifies warfarin and phenprocoumon among the medicines whose concentration or effect St John's Wort reduces, and attributes it to induction of CYP2C9 alongside CYP3A4 and CYP1A2 (Henderson 2002). A 2021 systematic review of 78 foods, herbs and supplements reported to interact with warfarin includes St John's Wort on its watch list, without stating a direction or size for it specifically (Tan 2021). The evidence base here is spontaneous case reporting rather than controlled study, and no trial has measured the size of the shift.
How it is supposed to work
Warfarin is cleared largely by CYP2C9, and St John's Wort induces that enzyme along with CYP3A4. More clearance means less anticoagulation from the same dose, which shows up as a falling INR result.
Which way it runs
One-directional, and the direction is easy to get backwards. St John's Wort reduces warfarin's effect rather than increasing it, so the risk is a clot rather than a bleed. Phenprocoumon, the coumarin anticoagulant used in parts of Europe, is affected the same way.
What is still missing
The bruising people are told to watch for is the sign of the opposite problem. Here the number moves the other way, and a warfarin level drifting quietly down between routine tests produces no symptom at all until it produces a clot.
Sources
- St John's wort (Hypericum perforatum): drug interactions and clinical outcomesHenderson L, Yue QY, Bergquist C, Gerden B, Arlett P, 2002
- Warfarin and food, herbal or dietary supplement interactions: A systematic reviewTan CSS, Lee SWH, 2021
- Herb-drug interactions with St John's wort (Hypericum perforatum): an update on clinical observationsBorrelli F, Izzo AA, 2009