Just be aware that, if you are on a statin and some other
meds, grapefruit is specifically contra-indicated.

This is from a TGA report:
http://www.tga.gov.au/adr/aadrb/aadr0212.htm#1
Australian Adverse Drug Reactions Bulletin
Volume 21, Number 4, December 2002

Interactions with grapefruit juice

The serendipitous discovery in 1991 of the interaction of
grapefruit juice with drugs occurred when grapefruit juice
was used to mask the taste of ethanol in a study testing an
interaction between the dihydropyridine calcium channel
blocker felodipine and ethanol.1 It is now known that
grapefruit juice can interact with a number of drugs, the
basis of the interaction being the local inhibition of one
of the cytochrome P450 enzymes (CYP3A4) and P-glycoprotein
(Pgp) in enterocytes in the intestinal wall.2 It has been
shown that grapefruit juice does not affect hepatic CYP3A4.

Interactions with grapefruit juice have been most frequently
studied with the dihydropyridine calcium channel blockers
(CCBs) including felodipine and nifedipine. Significant
interactions have also been found for some of the HMG-CoA
reductase inhibitors (statins), particularly simvastatin but
possibly also atorvastatin; the benzodiazepines midazolam
and triazolam; as well as cyclosporin, saquinavir, and
cisapride. This is not an exhaustive list and there are a
number of other drugs with a potential for interaction which
have not been studied. A recent article in the Australian
Prescriber contains a more comprehensive list.3 The two most
important characteristics of the "target" drugs are
metabolism by gut wall CYP3A4 and/or Pgp and associated low
oral bioavailability.

ADRAC has received 14 reports describing possible
interactions with grapefruit juice. Most have involved
either the dihydropyridine CCBs (5) or statins (5). Three of
the reports with CCBs have involved amlodipine, an
interaction which is usually considered clinically
insignificant. Grapefruit juice can inhibit the metabolism
of target drugs and increase the amount of parent drug
available for absorption, which may result in an increase in
its pharmacological or toxic effects. For the CCBs, the
reports usually describe hypotension and related symptoms,
and for the statins, most reports describe myalgia and
associated effects.

Prescribers should be aware that there are several groups of
drugs that may interact with grapefruit juice and patients
taking these drugs should be made aware of the possibility.
It should also be noted that problems can arise from whole
grapefruit (as in four of the ADRAC reports), and that the
extent of the interaction can vary with different brands and
strengths of juice. It is believed that with the exception
of bitter Seville oranges, the interaction does not occur
with other citrus fruits.

Options for discussion with patients include:

   * Avoid grapefruit juice all together
   * Take medication with grapefruit juice every day (with
dose adjustment if necessary)
   * Separate grapefruit juice and medication by a minimum
of 2 hours

References

  1. Bailey DG, Spence JD, Munoz C, Arnold JMO. Interaction
of citrus juices with felodipine and nifedipine. Lancet
1991; 337: 268-269.
  2. Bailey DG, Malcolm J, Arnold O, Spence JD. Grapefruit
juice-drug interaction. Br J Clin Pharmacol 1998; 46:
101-110.
  3. McNeece J. Grapefruit juice interactions. Aust Prescr
2002; 25: 37 (comprehensive table available at
www.australianprescriber.com)

Cheers, Alan, T2, Australia.
d&e, metformin 2x500mg