Good question, and one that comes up often.
Eating grapefruit alters the activity of certain enzymes in your liver. Some medications require these enzymes to be activated and while others require them to be degraded and then eliminated from your body.
Grapefruit and its juice raise, unpredictably so, the amount of Lipitor in your blood. This effect is magnified even more if you ingest grapefruit at the same time you take your medication.
To be safe, you should eliminate eating grapefruit or drinking grapefruit juice from your diet if on Lipitor and other statins.
To your health,