It is felt that grapefruit products interfere with a protein that is required to breakdown simvastatin. As a result, the level of simvastatin increases significantly in the body if grapefruit is consumed while on the medication.
Thus, it is recommended that patients who are on simvastatin should not consume grapefruit products.
If the simvastatin medication has been discontinued totally, then the patient can consume grapefruit as soon as he/she wishes. But, again, while on the medication, patients should not consume any grapefruit products.