Grapefruit and grapefruit juice are healthful, providing enough vitamin
C, potassium, dietary fiber, and other nutrients to earn the American
Heart Association’s “heart-check” mark. That’s
the good news. The bad news is that grapefruit juice can interact with
dozens of medications, sometimes dangerously.
Doctors are not sure which of the hundreds of chemicals in grapefruit
are responsible. The leading candidate is furanocoumarin. It
is also found in Seville (sour) oranges and tangelos; although these
fruits have not been studied in detail, the guidelines for grapefruit
should apply to them as well.
Grapefruit’s culprit chemical does not interact directly with
your pills. Instead, it binds to an enzyme in your intestinal tract known
as CYP3A4, which reduces the absorption of certain medications.
When grapefruit juice blocks the enzyme, it’s easier for the medication
to pass from your gut to your bloodstream. Blood levels will rise faster
and higher than normal, and in some cases the abnormally high levels
can be dangerous.
Certain chemicals that grapefruit products and citrus fruits contain can
interfere with the enzymes that break down (metabolize) various
medications in your digestive system. As a result, more medication stays
in your body. This can increase the potency of your medication to
potentially dangerous levels, causing serious side effects.