Why is one not supposed to have grapefruit or grapefruit juice when on Lipitor?

Dr. Joshua’s Answer:

Because they compete against each other in the liver - meaning that grapefruit juice will cause the liver to metabolize (eliminate) less Lipitor, possibly leading to increased and even toxic levels of the drug in the body.

Lipitor (atorvastatin) levels have been shown to increase in the blood after drinking grapefruit juice. The recommendation is to avoid grapefruit juice, at least in large amounts.

Reference:
Lilja JJ et al.
Grapefruit juice increases serum concentrations of atorvastatin and has no effect on pravastatin.
Clin Pharmacol Ther. 1999 Aug;66(2):118-27.

This interaction has been shown to be especially significant with another statin, namely simvastatin. On the other hand, fluvastatin has a weak interaction with grapefruit juice, and pravastatin and rosuvastatin do not significantly interact with grapefruit juice.