Simvastatin and grapefruit should not be combined because the presence of grapefruit in a person's system makes it hard to break down the active chemical in simvastatin. This can lead to a situation in which there is too much simvastatin in a person's body or where the drug remains in the body for too long. Either of these situations can lead to serious health complications. In most cases, it is safe to drink grapefruit juice or to eat grapefruit in small amounts while taking simvastatin.
Though the exact reasons that simvastatin and grapefruit should not be combined are not well understood, scientists have observed that patients who consume large quantities of the fruit have difficulty breaking down simvastatin. This chemical is metabolized by enzymes in the patient's body, but the presence of certain molecules found in grapefruit is thought to prevent these enzymes from breaking down the simvastatin. The drug remains in the patient's body for longer periods of time because it is not metabolized at the normal rate. This means that a normal dosing regimen of simvastatin may lead to a situation in which there is too much simvastatin in a patient's body at once.
When the grapefruit enzymes finally leave the patient's system, the simvastatin can be broken down in the proper manner. Unfortunately, because the drug was not broken down within the recommended amount of time, large amounts of it may persist in the person's body. If the molecules from a few doses of simvastatin are all broken down at once, an overdose can occur, even if the patient took the medication at the proper times. Unexplained pain in the muscles or joints, painful urination, dizziness, and loss of consciousness are possible signs of too much of this drug in the patient's body.
In order to avoid the risk of a potentially serious interaction between simvastatin and grapefruit, patients taking this medication are advised not to eat much grapefruit during treatment. Drinking large amounts of grapefruit juice and taking grapefruit extract or supplements can also cause an interaction between simvastatin and grapefruit. In most cases, however, it is safe to drink up to a liter of grapefruit juice or to eat the equivalent in whole fruit each day while taking this medication. Consuming even small amounts of grapefruit, however, can lead to a dramatic increase in the amount of simvastatin in a person's body and should be done cautiously.