Grapefruit has a high volume of compounds called furanocoumarins, which are designed to protect the fruit from fungal infections. When you ingest grapefruit, th

Grapefruit has a high volume of compounds called furanocoumarins, which are designed to protect the fruit from fungal infections. When you ingest grapefruit, those furanocoumarins take your cytochrome P450 enzymes offline. There’s no coming back. Grapefruit is powerful, and those cytochromes are donezo. So the body, when it encounters grapefruit, basically sighs, throws up its hands, and starts producing entirely new sets of cytochrome P450s. This can take over 12 hours. This rather suddenly takes away one of the body’s main defense mechanisms. If you have a drug with 10 percent bioavailability, for example, the drugmakers, assuming you have intact cytochrome P450s, will prescribe you 10 times the amount of the drug you actually need, because so little will actually make it to your bloodstream. But in the presence of grapefruit, without those cytochrome P450s, you’re not getting 10 percent of that drug. You’re getting 100 percent. You’re overdosing. And it does not take an excessive amount of grapefruit juice to have this effect: Less than a single cup can be enough, and the effect doesn’t seem to change as long as you hit that minimum. None of this is a mystery, at this point, and it’s shockingly common. Here’s a brief and incomplete list of some of the medications that research indicates get screwed up by grapefruit: Benzodiazepines (Xanax, Klonopin, and Valium) Amphetamines (Adderall and Ritalin) Anti-anxiety SSRIs (Zoloft and Paxil) Cholesterol-lowering statins (Lipitor and Crestor) Erectile-dysfunction drugs (Cialis and Viagra) Various over-the-counter meds (Tylenol, Allegra, and Prilosec) And about a hundred others.