Grapefruit & grapefruit juice contains a high level of acid that could breakdown the medication quicker than its supposed to. Most meds are meant to work over a period of time. Grapefruit would shorten that amount of time dramatically.
It is thought that one or more of the chemicals (most likely flavonoids) in grapefruit juice alter the activity of specific enzymes (such as CYP3A4 and CYP1A2) in the intestinal tract. These enzymes decrease the amount of certain drugs which enter the systemic circulation. This could allow a larger amount of these drugs to reach the bloodstream, resulting in increased activity and possibly toxicity.
Some of the drugs which interact with grapefruit have a narrow therapeutic index. This means that the amount needed for the desired effects are not much lower than the amount that can cause toxicity. In this type of medication even a moderate increase in blood levels could cause harm.
The reason why grapefruit juice is a problem for people taking certain drugs - including many psychotropic meds - is that grapefruit juice (unlike any other citrus juice) inhibits two enzymes that are important in metabolizing, or breaking down, these medications. When the enzymes aren't doing their job, much more of the drug in question remains available to enter the patient's bloodstream, and in some cases, this can have dangerous, even toxic, effects.
For example, as much as 99% of BuSpar (Buspirone) is normally metabolized before the drug ever enters the bloodstream. When taken in the presence of grapefruit juice (not even at the same time), the amount of BuSpar in the system could increase as much as 400% - a hazardous situation.
... investigators have found that the effect is measurable for at least 24 hours and may linger up to two days." This could mean that if a person drank grapefruit juice every day, there would be an extra-strength "grapefruit effect" in his system all the time, as the effect of yesterday's glass might still be lingering when today's glass was drunk.
Besides the ones listed above, psychotropic medications that can be affected to a greater or lesser degree by grapefruit juice include:
Anafranil
Clozaril
Elavil
Halcion
Haldol
Luvoxx
Seroquel
Serzone
Trazodone
Valium
Versed
Zoloft
There are a large number of other types of medications affected by grapefruit juice as well, including all the cholesterol-lowering statins and even some antibiotics and birth control pills.
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