Believe it or not, drinking grapefruit juice can be harmful to your health if you're taking certain medication. Drinking grapefruit juice together with certain medications can increase blood levels of the drug to harmful levels. The juice inhibits a chemical in the liver needed to break down many drugs in the body. The absence of this chemical can lead to high blood level, which can actually make a drug more potent.
This effect has been observed in nearly all calcium channel blockers, a group of drugs used to control blood pressure. Heart patients taking calcium channel blockers or cholesterol-lowering statins should talk to their physician about possible food-drug interactions.