Is a diet with a lot of grapefruit bad in some way?

I love grapefruit. Grapefruit juice, grapefruit salad, grapefruit fruit… Then I heard from a relative that something in grapefruits could conflict with some sorts of medicine. Is this true, and if it is, what kinds of medications does it screw up, or in fact, does it have anything to do with meds at all??