St John's wort (Hypericum perforatum) is predicted to reduce the blood levels of
phenobarbital.
The CSM in the UK advise that St John's wort should be stopped and that the
phenobarbital dosage should be adjusted. However, note that this prediction also covered
carbamazepine, which has subsequently been found not to interact, so a clinically significant interaction with
phenobarbital now seems less likely.