The problem: that certain "individuals have different levels of an enzyme in the intestines and liver, called CYP3A4, that breaks down drugs before they even have the chance to get into the bloodstream. People with very active CYP3A4 get lower amounts of drugs into their systems than those with low levels of the enzyme."
The solution: "But powerful compounds in the grapefruit called furanocoumarins obliterate CYP3A4 in the gut. The result: More drug gets into the bloodstream. For some anticholesterol statins, for example, taking one tablet with a glass of grapefruit juice "is like taking at least 10 tablets with a glass of water," says David Bailey, a pharmacologist at the University of Western Ontario who discovered the grapefruit effect in the early 1990s." With certain drugs (Lipitor, for example) you don't want that effect; for others (in this article, a weak anticancer compound) it makes all the difference.
It does have a flip side! The completely opposite effect...
Meanwhile, the grapefruit continues to surprise the scientific community. Recently, another class of compounds in the fruit was found to block a different set of proteins in the intestine known as "transporters." These transporter proteins actively shuttle drugs from the gut into the bloodstream. Blocking these transporters prevents some drugs from entering the system. This finding may mean that grapefruit is contraindicated with certain drugs for a whole new set of reasons.
Moral: Don't try this at home. Drink your grapefruit juice and take your meds, but not together, at least for now.