It ought to be better known than it is, but grapefruit juice interferes with the the working of some medication, in particular statins.
I take statins and it is listed in the leaflet that I mustn't have grapefruit juice, and indeed I have been advised not to have any grapefruit at all (of course, the minute someone says that I have a craving for a large pink grapefruit!).
I mention this because I have come across quite a few people who ought to have been told this but weren't. Lecture over!