Grapefruit is full of a type of organic compound called furanocoumarin, which interferes with the activity of an enzyme in your small intestine called CYP3A4. Problem is, that interference means your body will absorb more of certain medicines, for things like high cholesterol, high blood pressure, and anxiety, than it’s supposed to. This enzyme’s normal job is to chemically change certain potentially dangerous compounds before they can get to your bloodstream or liver. That way, they’re easier for your body to eliminate. But it also recognizes lots of different medications, and deactivates them the way it would any chemical — meaning that a large amount of the drug you take never actually makes it into your body to do its job.