Grapefruit juice is known to have various interactions
with drugs, in ways that are difficult to predict.
It's best to avoid it completely when using
prescription medications.

Geriatrics. 2006 Nov;61(11):12-8.

Grapefruit juice and drug interactions. Exploring
mechanisms of this interaction and potential toxicity
for certain drugs.
Bressler R.
Department of Medicine, College of Medicine,
University of Arizona, Arizona Health Sciences Center,
AZ, USA.

Concomitant administration of grapefruit juice can
increase the plasma concentration of numerous drugs
in humans and decrease the concentration of a few
others. Such elevations of drug plasma concentrations
have, on occasion, resulted in adverse clinical
effects. Increased concentrations are primarily
mediated by chemicals in grapefruit juice, which
inhibit the CYP 3A4 drug-metabolizing enzyme in the
small intestines. This inhibition decreases the
first-pass metabolism of drugs using the CYP 3A4
intestinal system and increases the bioavailability
and maximal plasma drug concentrations (Cmax) of
the CYP 3A4 substrates. The effect of grapefruit
juice on drug metabolism is most pronounced in
drugs with a high first-pass metabolism (eg,
felodipine, amiodarone), in which it inhibits
the first-pass metabolism of the CYP 3A4 substrates
leading to an increase in Cmax and area under the
concentration time curve (AUC). The use of
grapefruit juice with a few specific drugs
(eg, fexofenadine, digoxin) may lower plasma drug
concentrations by inhibiting drug absorption
catalyzed by the organic anion transporting
polypeptide (OATP).

J Clin Pharmacol. 2006 Dec;46(12):1390-416.
Grapefruit-drug interactions: can interactions
with drugs be avoided?
Mertens-Talcott SU, Zadezensky I, De Castro WV,
Derendorf H, Butterweck V.
Department of Pharmaceutics, Center for Food Drug
Interaction Research and Education, JHMHC, POB 110494,
Gainesville, FL 32610-0494.

Grapefruit is rich in flavonoids, which have been
demonstrated to have a preventive influence on many
chronic diseases, such as cancer and cardiovascular
disease. However, since the early 1990s, the potential
health benefits of grapefruit have been overshadowed
by the possible risk of interactions between drugs
and grapefruit and grapefruit juice. Several
drugs interacting with grapefruit are known in
different drug classes, such as HMG-CoA reductase
inhibitors, calcium antagonists, and immunosupressives.
Currently known mechanisms of interaction include the
inhibition of cytochrome P450 as a major mechanism,
but potential interactions with P-glycoprotein and
organic anion transporters have also been reported.
This review is designed to provide a comprehensive
summary of underlying mechanisms of interaction
and human clinical trials performed in the area of
grapefruit drug interactions and to point out possible
replacements for drugs with a high potential for
interactions.

Am Fam Physician. 2006 Aug 15;74(4):605-8.
Management of grapefruit-drug interactions.
Stump AL, Mayo T, Blum A.
University of Alabama School of Medicine, Tuscaloosa,
Alabama, USA.

Grapefruit is a healthy addition to a well-balanced diet.
However, the fruit has been shown to affect the metabolism
of many medications, increasing the risk of toxicity and
adverse effects. Characteristics of oral medications that
may interact with grapefruit include extensive metabolism
through the intestinal cytochrome P450 3A4 system, low
bioavailability, and a narrow therapeutic index. Prominent
medications known to interact with grapefruit include
statins, antiarrhythmic agents, immunosuppressive agents,
and calcium channel blockers. There are equally effective
alternatives to these drug classes that do not have the
potential to interact with grapefruit. These alternative
drugs may be substituted if a patient experiences or is
at risk of a grapefruit-drug interaction. Patients also
may choose to exclude grapefruit from their diets and
consume other fruits, including other types of citrus,
to avoid an interaction.

Grapefruit and grapefruit juice have interactions with about 50% of
all drugs, they inhibit the enzymes needed to metabolize these drugs
and therefore enhance their effects, which is equivalent to overdose
and could lead to dangerous situations. Avoiding grapefruit and
grapefruit juice at least say 2-3 hours before and after taking the
drugs is sensible, but I'm not sure, whether that is long enough.