Why many prescription medications include the
warning, “do not take with grapefruit juice.”
The reason is because grapefruit juice messes with your body’s
ability to properly metabolize the medicine. It blocks a crucial
enzyme (CYP3A4) that controls how much of the medicine you
receive. Drinking even one glass of grapefruit juice throws off
the safe dose. Basically, it is subject to a potentially
dramatic increase in systemic exposure and associated higher
risk of overdose with grapefruit as a result of diminished the
CYP3A4 activity, primarily in the small intestine, rather than
in the liver.