grapefruit and grapefruit juice have been found to interact with numerous drugs, in many cases resulting in adverse effects. this happens in two ways: one is that grapefruit can block an enzyme which metabolizes medication, and if the drug is not metabolized, then the level of the drug in the blood can become too high, leading to an adverse effect; the other effect is that grapefruit can block the absorption of drugs in the intestine, and if the drug is not absorbed, then not enough of it is in the blood to have a therapeutic effect.