I've just watched a program on UK's BBC2 - 'Trust Me I'm a Doctor' - which presented some startling information.
Apparently if you have a grapefruit (or juice) it interferes with the concentration of statins in the blood. If you take one with your breakfast it's the equivalent of ingesting 13 tablets, which is rather bad news.
Also, liquorice interferes with blood pressure medication.
Worth a heads up and a bit of research if you think it applies to you.