St John's wort is predicted to reduce the blood levels of
phenytoin. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
The CSM in the UK advise that St John's wort should be stopped and that the
phenytoin 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
phenytoin now seems less likely.