Grapefruit contains a chemical that inhibits a gut enzyme that breaks down certain medicines. Doses are calculated taking into account the fact that some of the drug will be broken down by this enzyme before it can act. So, if you've eaten grapefruit you may get an overdose of the drug because your gut isn't breaking any of it down. I understand that the grapefruit chemical is also present in Seville oranges. It is absent from other citrus fruits. You can find lists of medicines that are affected online. Most medicines aren't affected, but better safe than sorry, eh!

I can live without grapefruit but not without Seville orange marmalade...