It is on the warfarin warning lists, but no trial has ever measured it
It sits on a systematic review's watch list of 78 foods, herbs and supplements reported to interfere with warfarin, and the UK regulator records a reduced blood-thinning effect.
What the study found
The warfarin warning rests on case reports, not on a single trial
St John's Wort sits on the current systematic review's watch list of 78 foods, herbs and supplements reported to interfere with warfarin, and the UK regulator records the direction as a reduced blood-thinning effect. No controlled study has ever measured the size of it.
The caveat · Listed without size or direction
The review naming it does not state the size or even the direction of the effect for St John's Wort specifically, and its classification of 78 items leans heavily on case reports of varying quality. Warfarin is monitored by routine blood test, so the practical risk is a result drifting between checks rather than a failure nobody sees.
What we don't know
The review naming it does not state the size or even the direction of the effect for St John's Wort specifically, and its classification of 78 items leans heavily on case reports of varying quality. Warfarin is monitored by routine blood test, so the practical risk is a result drifting between checks rather than a failure nobody sees.